Taysha Gene Therapies Community letter

25/09/2021

September 24, 2021

Dear Rett Syndrome Community,

During the September 22 educational event on Rett syndrome for the investment community,
Taysha mentioned that, pending submission, review and approval from regulatory agencies, our
goal is to enroll patients ages 18 and over in the first planned clinical trial for TSHA-102, our
investigational gene therapy for Rett syndrome.

We must work closely with regulatory agencies to advance TSHA-102 towards an Investigational
New Drug (IND) application in the US and Clinical Trial Applications (CTA) outside of the US.
These application processes and regulatory approvals are required before any clinical trials can
begin. If the IND and CTA applications are accepted, we will have permission to start a clinical
trial in patients. We plan to share more information about the clinical trial at that time.

We are also pleased to share that Taysha has been granted Orphan Drug Designation (ODD)
from the European Commission for TSHA-102 for the treatment of Rett syndrome. The
European Commission grants orphan designation for medicines intended to be developed for
the diagnosis, prevention or treatment of disorders affecting fewer than five in 10,000 people
in the European Union. TSHA-102 was also previously granted both Rare Pediatric Disease
Designation (RPDD) and Orphan Drug Designation (ODD) from the US Food and Drug
Administration (FDA).

These designations are not associated with the start of clinical trials and do not represent any
regulatory approval or validation of our approach. However, this is an important step and
milestone to emerge from our Rett syndrome program. For more information regarding this
designation and TSHA-102, please read our press release.

We are committed to the Rett syndrome community and are grateful for the support from
advocates and caregivers. We look forward to continuing to provide you with updates related to
Taysha’s Rett syndrome program through our Rett syndrome advocacy partners.

Your Rare Ally,

The Taysha Team

This communication contains forward-looking statements within the meaning of the Private Securities Litigation
Reform Act of 1995. Forward-looking statements are based on Taysha’s current expectations and are subject to
various risks and uncertainties that could cause actual results to differ materially and adversely, including those
described in our filings with the Securities and Exchange Commission , which is available at www.sec.gov. These
forward-looking statements speak only as of the date hereof, and we disclaim any obligation to update these
statements except as may be required by law.