Good afternoon and welcome to the Taysha Gene Therapies Third Quarter 2023 Earnings Call. At this time, all participants are in a listen-only mode. A question-and-answer will follow the formal presentation. [Operator Instructions] Please note that this call is being recorded. I would now like to turn the call over to Hayleigh Collins, Director, Head of Corporate Communications and Investor Relations. Thank you and you may proceed, Hayleigh.

Thank you. Good afternoon and welcome to Taysha’s third quarter 2023 financial results and corporate update conference call. Earlier today, Taysha issued a press release announcing financial results for the third quarter 2023. A copy of this press release is available on the Company's website and through our SEC filings. Joining me on today's call are Sean Nolan, Taysha's CEO; Sukumar Nagendran, President and Head of R&A; Kamran Alam, Chief Financial Officer. We will hold a question-and-answer session following our prepared remarks. Please note that on today's call, we will be making forward-looking statements including statements relating to the therapeutic and commercial potential of TSHA-102 including the reproducibility and durability of any favorable results initially seen in our first and second patients dosed in the REVEAL trial and including our preclinical product candidates to positively impact quality of life and alter the course of disease in the patients we seek to treat in our research, development and regulatory plans for our product candidates. These statements may include the expected timing and results of clinical trials for our product candidates and other clinical and regulatory plans, and the market opportunity for those programs. This call may also contain forward-looking statements relating to Taysha’s growth, forecasted cash runway in future operating results, discovery and development of product candidates, strategic alliances and intellectual property, as well as matters that are not historical facts or information. Various risks may Taysha's actual results to differ materially from those stated or implied in such forward-looking statements. These risks include uncertainties related to the timing and results of clinical trials and regulatory interactions for our product candidates are dependents upon strategic alliances and other third party relationships, our ability to obtain patent protection for our discoveries, limitations imposed by patents owned or controlled by third parties, and the requirements of substantial funding to conduct our research and development activities. For a list and description of the risks and uncertainties that we face, please see the reports that we have filed with Securities and Exchange Commission, including our annual report on Form 10-K for the year ended December 31, 2022, and our quarterly report on Form 10-Q for the quarter ended September 30, 2023 that we filed today. The conference call contains time-sensitive information that is accurate only as of the date of this live broadcast, November 14, 2023. Taysha undertakes no obligation to revise or update any forward-looking statements to reflect events or circumstances after the date of this conference call, except as may be required by applicable securities laws. With that, I would now like to turn the call over to our CEO, Sean Nolan.

Thank you, Hayleigh, and welcome everyone to our 2023 third quarter financial results and corporate update conference call. Today, I will begin with a brief update on our corporate and clinical activities. Then Dr. Sukumar Nagendran, President and head of R&D of Taysha will provide an update on the clinical development of our TSHA-102 program. Kamran Alam, our Chief Financial Officer, will follow up with a financial update and I will provide closing remarks and open the call up for questions. This quarter, we continue to advance the clinical development of our lead gene therapy program in Rett syndrome, including generating new clinical data in our adult trial in Canada and further engaging in regulatory discussions on our planned pediatric trials in the United States and United Kingdom. As a reminder, the REVEAL Phase 1/2 adult trial is a first in human study that was designed primarily to evaluate safety. Recall, when we initiated the REVEAL trial in Canada, there was low expectation of efficacy for the stage 4 adult population among the KOLs in the Rett syndrome community, due to the advanced and relentless progression of the disease. The focus was placed primarily on safety. Therefore, it was very exciting when we announced the encouraging initial impact that TSHA-102 appeared to have across multiple clinical domains in the first adult patient treated. Today, we are pleased to share that as of the week 12 assessment, patient one has demonstrated a sustained response and key efficacy measures and new improvements in several areas, including hand function, which is a hallmark manifestation of Rett syndrome. Additionally, the second adult patient treated also demonstrated a consistent early response across multiple clinical domains four weeks following treatment. The two patients have quite different genetic mutations, with the first patient's MECP2 mutation manifesting in…

Thank you, Sean, and good afternoon everyone. I'm pleased to provide an update on our TSHA-102 gene therapy program in clinical evaluation for the treatment of Rett syndrome. As a reminder, TSHA-102 utilizes a novel MRI technology designed to mediate MECP2 expression in the central nervous system on a cell by cell basis to mitigate risk of over expression the X chromosomal inactivation and silencing of MECP2 expression that occurs randomly with Rett syndrome results in a mixture of cells that are either deficient in or express MECP2 normally. The heterogeneity in MECP2 expression is what makes Rett syndrome challenging with traditional small molecule and simple gene therapy approaches, but we believe our novel MRI technology can appropriately address this challenge and provide therapeutic benefit. TSHA-102 is currently being investigated in the ongoing REVEAL Phase 1/2 adult trial of first in human open label, randomized dose escalation and dose expansion study, evaluating the safety and preliminary efficacy of TSHA-102 in adult females with Rett syndrome due to me P2, loss of function mutation. The trial, which was designed primarily as a safety study, is also measuring pre-specified efficacy measures. All efficacy data being collected in this Phase 1/2 trial is hypothesis generating as we continue to generate longer term data across more patients and cohorts, these measures will further inform our thinking relative to optimal primary endpoint selection for registration study purposes. To date, two adult patients have been dosed with TSHA-102 in the first cohort evaluating the low dose. We are highly encouraged by the early safety and efficacy data seen in both adult patients. Importantly, we believe these data which have been reviewed by the IDMC, reinforce the transformational potential of TSHA-102. TSHA-102 was generally well tolerated with no treatment emergency areas, adverse events as of 20 week…

Thank you, Suku. Research and development expenses were $11.8 million for the three month ended September 30, 2023 compared to $16.8 million for the three month ended September 30, 2022. The net change was due to a $9.3 million decrease due to lower compensation expense as a result of reduced headcount, lower licensing milestone fees, fewer manufacturing batches, and fewer raw material purchases. This was partially offset by a $4.3 million increase in activity surrounding ongoing clinical trial efforts and the Rett syndrome REVEAL adult and pediatric studies. General and administrative expenses were $8.6 million for the three months ended September 30, 2023 compared to $8.7 million for the three months ended September 30, 2022. The decrease of $0.1 million was due to reduced compensation expense due to lower head count of $2 million and reduced consulting and professional fees of $0.7 million, partially offset by $2.6 million issuance costs allocated to the liability classified pre-funded warrant issued in connection with the private placement financing completed in August, 2023. Net loss for the three months ended September, 2023 was $117.1 million or $0.93 per share as compared to a net loss of $26.5 million or $0.65 per share for the three months ended September 30, 2022 due to a $100.5 million non-cash expense recorded in Q3 2023 from a change in the fair value of warrant liability from pre-funded warrants in connection with the private placement financing completed in August 2023. As of September 30, 2023, the Company had cash and cash equivalent of $164.3 million. The Company expects that its existing cash and cash equivalent will fund operating expenses and capital requirements into 2026. Taysha entered into a loan and security agreement with Trinity Capital on November 13th and terminated its existing loan and security agreement with Silicon Valley Bank. As Sean noted, our cash runaway now extends into 2026 as a result of this agreement and there are no financial covenants or warrants associated with the loan and security agreement. I'll now turn the call back over to Sean for his closing remarks. Sean?

Thank you, Cameron. We are highly encouraged by the clinical data observed in TSHA-102 program to date, the initial safety profile of TSHA-102, and the early and consistent pattern of improvement in observed across clinical domains in both adult patients with very different genetic mutation severity, coupled with the sustained response through week 12 for the first patient is encouraging. We believe the data presented today further validates the therapeutic potential of TSHA-102 for patients and families living with Rett syndrome. We remain focused on our key upcoming milestones as we approach 2024, including dosing additional patients in the REVEAL adult trial in both low and high dose cohorts and expanding our clinical footprint into pediatric patients in the U.S. and UK. With that, I will now ask the operator to begin our Q&A session. Operator?

[Operator Instructions] The first question comes from Whitney Ijem from Canaccord Genuity. Please proceed with your question, Whitney.

First question from me, again, great data. Just curious though, in terms of the RSVQ, it looked like in the first patient it increased slightly from week 8 to week 12. Sorry if I missed it, but can you provide any more color, I guess, on which domains in particular changed over that timeframe?

Whitney, I can turn it over to Suku, but I think, the main domains where there were changed were in breathing socialization and gross and fine motor skills. Would you -- is there anything that you would add to that Suku?

No, Sean, I mean, as you pointed out, those were the domains that actually improved and I think what is interesting to note is that it also correlates with some of the clinical observations that are obvious that have been evaluated by the PI as well as evaluated when it comes to the broad application of CGI and CGIS. So, it's an interesting correlation and given that patient one month’s much sicker than patient two, you also see the significant change there in the RSVQ that is maintained from 4 weeks to 12 weeks post-treatment.

And then just does. And then just kind of a broader question, how are you guys thinking about these two patients from a proof-of-concept perspective for the miRARE tech? I guess kind of what else or what else in terms of data or patients, patient number or follow-up would you be looking for to kind of consider this technology de-risked? And can you remind us how you're thinking about additional indications where it might make sense?

Sure. Well, I'll go first and certainly open up the Suku, if he has additional comments. But I would say, you know, what's encouraging to us about, you know, proof of concept with the miRARE technology is that the first patient had significantly severe disease at baseline, right? I mean, the CGIS was at 6. So you would infer from that that she's got a relatively low level of endogenous MECP2 and yet preliminarily with through we week 12, we're seeing encouraging response across clinical domains. And then you juxtapose that with patient two. So keep in mind that that phenotype from patient one is based on a significant deletion genetically. Patient two had a missense mutation resulting in a much less severe manifestation of the disease. So you would think she probably is generating more endogenous MECP2 than patient one. And yet, we through week, let's see, week six, haven't seen any safety concerns with that particular patient. So in a way, Whitney, I think with the first two patients seen, you're seeing very, very different genotypes corresponding phenotypes and levels of endogenous MECP2 and you're seeing consistent response across clinical domains, and there appears to be a promising safety profile. So, I would say these first two patients are good indicators of the potential safety of the miRARE technology.

And what I would add to what you just said, Sean, in very simple terms, is given that there were two different genotypes with different phenotype representations that fit into the classification of classic Rett syndrome, it is reassuring to see that our gene therapy actually worked quite effectively in both patients. And as we continue to accumulate more data and accumulate different genotypes that present as classic Rett syndrome, it'll also give us the confidence that this gene therapy will be applicable to the broader Rett syndrome patient population as a whole. And the second piece of the puzzle is, as you pointed out or you asked, miRARE technology is critical to maintain a very tight control of MECP2 protein levels in the nucleus. And I think based on the clinical improvements that appear to happen very quickly post gene therapy, given that our product has self-complimented technology as well, that changes have been noted in a positive manner by the PI within one to two weeks post dosing. And this is also reassuring because it enables us to see clinical improvements, which also precede the need for the biomarkers, as such to measure response, which is sometimes what you need in other products, because they take much longer to act.

The next question comes from Kristen Kluska from Cantor Fitzgerald. Please proceed with your question, Kristen.

Let me also add my congratulations that a second patient exceeded the expectations here. With this particular readout, you commented a lot about the cardiac and some of the respiratory items. And I wanted to see if you can kind of give us a context about the importance of some of these measures. From our understanding is with this disorder, about a quarter of the patient's due experience, a sudden death as a result of complications from these matters.

Yes, let me address that because that's a very important question. So, Rett syndrome patients typically do have autonomic dysfunction. And when you think of autonomic dysfunction, it's usually a respiratory abnormalities i.e., they have episodes of apnea where they have breath holding spells followed by significant hyperventilation. That's one. Second, they have a history of seizures depending on which part of the brain, the foresight exists. They also have significant sleep abnormalities at times where they cannot fall asleep, cannot stay asleep, may have also sleep terrors, and they also have GI dysfunction usually I think it's more constipation than diarrhea. So, these are pretty standard when it comes to Rett syndrome patients. And what we highlighted in patient one and patient two is that our gene therapy, once dose within a week or two, had significant positive impact on these respiratory abnormalities. So, there was significant reduction in these apnea spell as well as hyperventilation episodes. We did not really talk about cardiac abnormalities because these patients did not have any cardiac abnormalities in rhythm or any concerns on the EKGs done. So, those are not clinically appropriate at this point in time for an efficacy measure of a clinical evaluation with our gene therapy.

Okay, thanks. And then looking at the two different phenotypes here, do you think that the scales and the specific anecdotes shared today weigh differently across the different types? Or is it more important you think to just focus on the bigger picture? So I guess while, all of these endpoints you shared are important, I guess the question I'm trying to ask is if any of them are particularly weighed more for each of the different phenotype shared. Thanks again.

Yes, let me take that question, that's another very important question because when you look at Rett syndrome patients, you have to break them down into what is their clinical abnormalities that they present with, what are the autonomic dysfunction abnormalities they present with and then these scales that we refer to CGI-I, CGI-S, RSBQ, R-MBA and the hand function scales. I would really focus on the clinical impact of our gene therapy. Because, for example, if you looked at patient one and patient two, even though their phenotypes are different, they both had developmental milestone abnormalities, meaning patient one was unable to sit, I think after the age of seven or eight. Patient two had difficulties in walking in an appropriate maintenance of gait and posture, probably after the age of 10. These patients also have difficulty with sometimes tracking objects with their eyes. Most of them do have significant hand function abnormalities and stereotypic movements, and they also have difficulties with social interaction and many of them have cognitive challenges. So clinically, those are components that we have to focus on. And we did highlight that our gene therapy in both patient one and two did have impact on some of these aspects of the disease. We've also highlighted the impact on the autonomic dysfunction and then the scales that you refer to really the numbers on the scales and the degree of change on these scales actually depend on how severe the phenotype is. The more severe the phenotype, the higher the RSBQ and the RMB scores can be. And then post-treatment, if the therapeutic is efficacious then you'll see a relative change in the number and you will see that even in RSBQ for the second patient, the numbers are much lower because the patient overall has a much milder phenotype. So, you have to keep that relative aspect in mind and look at the broader clinical impact of the product because clinical transformation impact, I think, is far more important from a clinical management and regulatory review standpoint sometimes than a certain scales. And I'll leave it at that for now.

The next question comes from Salveen Richter from Goldman Sachs.

This is Elizabeth on for sine. Congratulations on the data. I was hoping you could provide additional color on the seizure activity in the seizure diary. Specifically what was the baseline seizure rate for each patient prior to steroid treatment and what's being observed now? And then also for the second patient, if you could provide some color on how quickly improvements were observed. I know for the first patient, those were observed quite soon after treatment. So any color there would be helpful.

Yes. So, that's a very important question that you ask because, so I'm going to try and walk you through first patient and second patient because there are lots of clinical aspects to the seizure history in both patients. So patient one, stage four 20 year old female, right, with severe Rett syndrome had four to five seizures per quarter before the gene therapy was given. The first six weeks post gene therapy there were no reported seizures in the seizure diary nor in the EEG that was reviewed by the PI. After week six, what was noticed is, this patient is on prednisolone and sirolimus and sirolimus actually works on the liver and inhibits the cytochrome P450 system, which results in Dilantin levels dropping significantly. So what happened after week six up to now is that the Dilantin levels dropped to below 50, which is very low, and therefore there was some breakthrough seizures in this patient. From what I recall, I think this patient had about seven seizures during the period following the six weeks post-treatment. Now, it is also of interest to note before the gene therapy was given, whenever this patient one had seizures, usually it was when the Dilantin levels were below 100, but I think usually above 50. So I would say that even though this patient had breakthrough seizures six weeks post-gene therapy, this happened when the Dilantin levels were below 50. And therefore, I think the gene therapy still had a protective effect, but the Dilantin levels got way too low to which allowed for breakthrough seizures. So that is patient one, and the patient is doing well. Patient…

Yes. So just to try to hit a punch line on that question, Elizabeth, I would say pre and post treatment through week 20 the seizure rates are comparable. His pre-treatment, the patient generally experienced seizures if she had dilantin levels below a 100. And now what it is looking like post-treatment is that the seizures occur when she's less than 50 so at a much lower amount of Dilantin.

Yes. Thank you, Sean, for clarifying that. And then patient two had roughly four seizures per month, which is 12 seizures, I guess every quarter. Following gene therapy treatment with our product, this patient had one breakthrough seizure up to week six, if I recall on day 13th. But that was it, just one seizure. So, that is a good signal that our gene therapy is most likely having impact on seizure incidence in this patient as well. So, that's the update on the seizure question for you. Thank you.

Super helpful. And then just on the second question of the rapidity of response observed in the second patient.

Yes. So, that's another important question. So just again to go back to the technology, jour product has a self-complimentary DNA. So once it's given into the central CSF and gets into the central nervous system, the episode is formed within 48 hours and we think starts producing the MECP1 protein. And the principal investigator has noted that clinical impact appears to be noted within seven to 10 days initially post the gene therapy being given via lumbar puncture. And this includes impact on some motor function as well as reducing some of the autonomic abnormalities. So we are pleased to see that and we hope that will continue because that will hopefully give some of these patients rapid relief of their serious symptomology.

Thank you. The next question comes from Gil Blum from Needham & Co. Please proceed with your question.

Allow me to have my congratulations. It's actually rather impressive. So one thing I do want to understand about some of these metrics, could there be a ceiling effect, I mean, specifically talking about RSBQ, we're kind of seeing a leveling around 30. I'm not super familiar with the scale, but is that a possibility?

A ceiling effect?

Yes, sorry, go ahead, Sean.

Well, what I would say to that, Gil, is a non-clinician would be -- I think we have to take into account the stage of the disease, the age of the patients and the dose that we're in at this particular point in time in the study all have to be considered into whether or not there's a ceiling being demonstrated. We are right now landing in some similar spots to your point, and it's still a little early in the assessment, but with that, I'll defer to Suku on the clinical question.

No, that's an important question that I think we are still struggling to assess, because keep in mind that both these patients were adults, right? So they were 20 and 21 year old females where we frankly did not expect to see much effect. And the most severe the patient, the higher, I assume the RSBQ score should be given that there are 45 items in that scale, and 38 of them are used to maximize the score of to up to a 90. And also keep in mind the -- it is very important to understand who actually does the assessment. It's usually done by the parents or the caregivers over a period of time. So they have to reflect on how the patient or their child or is doing over a course of a week or so. So there can be some variability in the score being assessed as well. And then finally, as far as there being a ceiling effect, I think what we are observing with the gene therapy is the more sicker the patient, the higher the score and the greater the drop. While the patient is not as sick, the drop may not be as significant because they're starting at a much lower baseline. I think I can probably answer your question once you dose a few more patients and we have a little more data. And then the other thing to keep in mind with RSBQ is that the questions you have to actually look at the questions being asked because some of the questions have been designed to evaluate the disease process that worsens over time. RSBQ frankly was not developed to evaluate the therapeutic intervention. Now, the caveat is for phenotype did use RSBQ combined with CGI-I for a combined endpoint to get their approval. And if you look at the 17 to 20 year old group that for phenotype study that drop in the RSBQ only 2 points. So I guess in relative terms, even though I don't like doing cross study comparisons, our first patient showed a 22 and a 23 point drop and the second patient who has much milder disease at a 4 point drop.

The severity was…

And the severity, that's right.

And I agree this is very early. This is more for educational purposes. My second question, so kind of as a segue in a hypothetical adult study, would it be beneficial to look at patients with more severe disease just to increase the signal to noise?

Yes, you're saying, basically enrich the study with more severe patients to show a bigger effect. You know what's interesting, Gil, when you look at the phenotype data, they generated their P-value difference essentially off the severe patients. But there really wasn't much effect in CGI-I or RSVQ and mild to moderate. So your point's a good one. I think, what's exciting us and we have to see more patients, I think to get to a more definitive answer. But what we liked about this was the fact that the genotypes were so different and the corresponding phenotypes were so different, yet we're still seeing response is very encouraging that, again, the population that could be treatable here continues to remain significant. So I would say, to reaffirm Suku's point, let's see some more patients, let's get to the high dose as well. And that'll certainly inform how we think about Part B endpoints and trial design.

The next question comes from Yanan Zhu from Wells Fargo Securities. Please proceed with your question.

So, I have a question on R-MBA and I think, if I heard this correctly, I think the patient number two's R-MBA improvement is roughly 7 points. And that sounds like, that's as big as patient number one's 12 week improvement and much bigger and bigger than patient number one four week improvement. If you comment on why despite of the milder disease in this patient R-MBA improvement appears to be quite striking? And if you can provide the baseline R-MBA score for this patient that would be great. And I have a maybe one or two follow ups.

So, the baseline scores to your point were for patient one was 43, for patient two is 38, and then at week four for patient two is 31, week four for patient one was 48, and now at week 12 patient one's at 37. So, there's been a 6 point drop in patient one over 12 weeks and a 7 point drop in patient two over four weeks. And then, I'll turn it over to Suku to try to get at the rest of your question. What was driving this?

Right. So that's an important question because R-MBA is a clinician assessment of the patient as well, and it has five components and I think goes up to a score of as you can see on the scale. The first patient who was much sicker, these R-MBA score was influenced by anxiety, right, and fear and also increased paroxysm, right, which shifted the score in a manner that didn't show improvement. But actually, when we talked to some of the experts, the understanding was that increased muscle function in the face included the maceta muscles where there bruxism is actually positive. But in the assessment of these questions that were set up, as I said, they were arranged and designed to actually show disease worsening, not necessarily a therapeutic response. And to my knowledge, R-MBA has never been validated to evaluate a therapeutic intervention. In patient two though, given that this patient had much milder disease, the change I think is positive because this patient already did not have some of the previous abnormalities of severe patients such as patient one hand. And therefore, the questions asked in the R-MBA didn't cloud the actual assessment. So, I guess my point again is that you need to really understand what the questions are within some of these measures to make sense of the eventual numbers. And Kamran, do you want to comment on that or?

So, in terms of provide the motor behavior assessment some of the improvements were in social skills and respiratory behavior.

So, I guess if I can add one punctuate that point though, what I would also say is that the clinical assessments of each of these patients, especially hopefully for a transformative therapy, will significantly overcome sometimes the need for these skills that mostly have not been validated for therapeutic intervention and assessment.

Thank you, and then -- I'm sorry, my follow up is that for the 4 point improvement RSBQ, which sounded like similar to Treg phenotype, what Treg phenotype can achieve, but it is also on the other hand, sounds like the patient had many improvements clinically. Could you comment on what could the improve the nature of the improvement differ in what you saw in this patient compared with if this patient had been treated with Treg phenotype for example? And maybe lastly, about patient number three, I was just wondering, have you identified that patient, if so, how severe that patient is? Thank you.

I would say, with the Treg phenotype piece of the question, when you look at patients that had moderate or mild disease, the impact on RSBQ was 1. So in this particular circumstance, you would definitely grade this person out as a mild or moderate and the impact that we demonstrated was 4 on that particular scale. And as we said, it was driven primarily by the breathing, the socialization and the gross and fine motor skills. So, it's very consistent with the feedback that that's being put forward by the physician and her assessments and observations by the parents and their assessments and observations. And where she's landing is 33, which is in the same ballpark at week 12 is where the first patient is landing, which is at 30. So, it looks like we're getting a very nice response from her. And again, I think the Suku’s point, the scales I think to me are becoming. It's good to see correlation that the scales are moving in the right direction along with what's being observed clinically. And we're seeing a lot of congruence across clinical domains, which I think is a testament to the product getting to where it needs to be levels of MECP2 being put forward into a more therapeutic area and which is why we're seeing some comprehensive behavior across the table. As it relates to patient number three, I would say that we have identified the next adult patient. She has not yet gone through screening and there's a couple things at play which is why we modified the guidance to Q4 this year. Q1 next year dosing is that it's final stages of an LAR process, which we've talked about in the past, which we don't have absolute line of sight to, so we know it should be in that window. And within the institution there's windows of time given the holidays and what have you -- where the patient can slot in. So, it is still possible this year could be Q1 next year. We'll keep you apprised on that, but we do not know any baseline characteristics at this particular juncture. We just do know that it would be a stage four patient.

The next question comes from Jack Allen from Baird. Please proceed with your question, Jack.

I was hoping you could provide some more context around the steroid regimen that's given post dose here and how the results from patient one or shaping up in the context of the steroid taper? So, what degree of confidence do you have that these results are different from potential? Any potential impact that could be due to steroids in the short term post gene therapy?

Yes, so for this protocol, all patients who are over the age of 18 will be started on prednisolone, 1 milligram per kilogram from day minus seven. And that will be continued out to 16 weeks and then it'll be tapered off over a certain number of weeks down to 0.5 milligrams a kilogram and then 0.25. These patients also get sirolimus at a much lower dose than what's used for transplant based on the body surface area. Also starting around day minus seven and runs, if I recall, 36 weeks and then it's tapered after that. We've looked extensively into the literature, we've talked to all the experts who treat Rett syndrome and they've all said that both prednisone and sirolimus no way or form impacts the clinical progression of the disease. Some people have actually done some experiments too, and they say there is no impact and therefore any improvement seen with the disease is actually due to the gene therapy that has been given.

And then just as it relates to the sirolimus dosing and the potential implications that had on the seizures the patient two, have you given any consideration to modifying that dosing moving forward?

Yes, we've actually have started looking into it, but the issue is sirolimus with prednisolone and we have some experience now with the GAN program where we showed that patients could be dosed regardless of the seropositivity to AAV9 antibodies, which is actually very helpful, especially given that our gene therapy is working in adult patients as well, where there is a higher seropositivity. But we have to balance that, as you said, with the type of anti-seizure medicine being used, especially if levels do drop significantly like what happened with Dilantin. So I guess the bigger question is, if -- are there other medicines that we can try, which are not processed in a way through the liver and impacted negatively by sirolimus. So I think that's something that we have to continue to assess for now.

Yes, I would just add to that, Jack, that it was patient one that was on the Dilantin. And the other thing, at least based on her history, is she's tried multiple products to control the seizures, and Dilantin was the only one that appeared to work. So, it may be a bit of a unique circumstance. We'll just have to see and monitor carefully as we proceed in the study.

Thank you. The next question comes from Joon Lee from Truist Securities. Please proceed with your question.

Congrats on the data and thanks for taking our question. This is Mandy for Joon. So you have a closed direct competitor in gene therapy or Rett syndrome from Neurogen NGN-401. Could you please elaborate on anything specifics in your program that could be viewed as a strong differentiator of your program? I mean from construct, design, route of administration those levels because the capsid seems to be the same. And I have a second question, which is related to TSHA-120 in GAN. So are there -- what's the latest there? Is there any takers or are there any negotiations going on for this program? Thank you.

Thanks, Joon. Well let's, let's start with the GAN question. I would just say there's not a lot to comment on specifically. We are in the process that we've outlined of stepping through strategic alternatives, and I wouldn't want to comment on anything until it's actually been resolved. So stay tuned on that one, but nothing to report specifically. As it relates to Neurogen, I guess a couple things I could say high level would be -- you're correct that both constructs use AAV9 as the capsid. But that's where the similarities end. So number one, we have a mini gene. They have a full-length gene. Promoters are slightly different. I think that a key aspect is really the regulatory element, miRARE that we have versus the exact technology that they have. So, the way I would describe our construct is that, it's able to take into account and read the endogenous levels of MECP2 in each cell that it goes into. And based on the level of endogenous MECP2, it will either create exogenous MECP2 or not. And so I think that's the distinguishing feature where our understanding of the exact technology is that essentially regulates itself. It is not taking into account the endogenous levels of MECP2. So potentially, there could be additional risk for over expression with that construct. I mean, we'll find out in the clinical setting. But that's a key area of differentiation. I would also just note that we're going through intrathecal delivery versus the ICV route that the Neurogen team is embarking upon there. So, there's a lot of differences. I can't comment anymore on the Neurogen program. I'm not aware of anything specific. But I would just say at this particular point in time, we're quite pleased with what we're seeing in our construct. And I would just go back to the first question that Whitney asked. And proof of concept with our construct with these two very different phenotypes and genotypes is encouraging, right? We saw a very severe patient that would arguably have very low levels of MECP2, and we're seeing a nice response. And in a patient with a less severe mutation, where you would expect her to be producing significantly more MECP2, we're seeing response coupled with the fact that we're not seeing any signs of over expression. So hopefully that gets at your question, Jim.

[Operator Instructions] The next question comes from Silvan Tuerkcan from JMP Securities. Please proceed with your question.

Thank you very much for taking my questions and congratulations on the great update here. Just considering this is one single trial site and one investigator, can you just comment on the objectivity and variability of especially autonomic function measures? I would assume that they're measured by some sort of scale a device. How does the breathing pattern, how is it measured, how does it improve? And also the sleep quality and duration. I think that's a very important measure here.

Yes, I would just say that there's a lot of -- both of those functions are measured via different scales and by different people. Some of them are going to be measured like the CGI-I as an example, the R-MBA, that's going to be measured by the clinician. That's her clinical observations of what's happening during a clinical assessment. Whereas with the RSVQ or the PGI, that's going to be caregiver administered, and they're going to log that in and demonstrate what they're seeing there. And then we do have examples like when we look at motor function like the hand function is a good example, right? That's being videotaped and sent to an independent party to review and score that. So Suku, I'll turn it over to you to add more color.

And also I think your question was with reference to autonomic abnormalities, right, seen in these patients. So, I'll break this down for you. So remember, in these adult patients who've been referred into this single center in Montreal, they do have physicians who've been taking care of them now for a very long time, so you have the medical history that allow as documents, the abnormalities seen, whether it's in sleep, seizure history, respiratory abnormalities or GI function. Also, when it comes to sleep, usually these patients also have sleep lab evaluations whenever appropriate and possible that documents the abnormalities that are observed. When it comes to seizures that are usually seizure diaries, parental diaries, and also EEG activities that have been measured that give you some sense of how often these seizures happen. And then when it comes to GI dysfunction, it depends on clinical reports, but also sometimes you do mobility studies in these patients, that may give you a good sense how abnormal GI function could be. So those are the general parameters that are used to make these assessments. And then the PI obviously does the assessments based on past history and then post-treatment evaluation for the protocol. And then finally, some of these questionnaires scales that Sean was referring to may have subtle questions or obvious questions that give you some hint of is, are there continued sleep abnormalities, are there continued demo motor dysfunction, are there continued respiratory abnormalities, et cetera. So it's a collective assessment of each patient over time.

And just regarding the dosing of the -- not the third patient, but then at the higher, going to the higher dose, is there another DSMB meeting required to switch the higher dose post dosing of the third patient? Thank you so much.

Yes. Once the third patient is dosed, you wait six weeks and then you collect the data, clean the database, and then you have to meet with the IDMC or DSMB before you go to the higher dose cohort of 1E15.

The next question comes from Eun Yang from Jefferies. Please proceed with your question. And it seems like we no longer have Eun connected on the line, and at this time there are no further questions in the queue. Ladies and gentlemen, we have reached the end of the question-and-answer session, and I'd like to turn the call back to Sean Nolan for closing remarks. Thank you, Sean.

Just want to thank everyone for attending the call. We appreciate the questions and look forward to speaking again soon. Take care.

Thank you. Ladies and gentlemen, that does conclude today's conference. Thank you very much for joining us. You may now disconnect your lines.