Dear Huntington's Disease Community,

We are excited to introduce ourselves and share a bit more about our history in Huntington's disease. Spark Therapeutics, a member of the Roche group, was founded on a belief in the transformative power of gene therapies, and our research in Huntington's disease goes back to some of our earliest pursuits as a company. Our research continues and is being carried forward by a team with extensive experience in Huntington's disease.

As part of our commitment to provide timely updates on our progress in Huntington's disease, we want to share news about our recent presentation of the "Innovator's Forum: at the Huntington Study Group's (HSG) 2024 Annual Meeting in Cincinnati (OH). Dr. Juha Savola, Vice President of Clinical Development and Clinical Leader for the Spark Huntington's disease program, presented an overview of our program to date. This presentation included a brief outline of our proposed clinical study design in late Stage 2 and early Stage 3 (Huntington Disease Integrated Staging System criteria) participants with Huntington's disease. We have not yet started the clinical study, and so the information shared by Dr. Savola could still change.

We are working with the HSG on a first-in-human study planned to open in 2025 and will be sending updates as appropriate when we have more information to share regarding our progress.

We would like to express our appreciation for the valuable work of the patient advocacy organizations, including Huntington's Disease Youth Organization (HDYO), Huntington's Disease Society of America (HDSA), Help4HD, HD Reach and the European Huntington's Association, and for their open communication and partnership. We want to thank those in the Huntington's disease community who have worked with us, shared their experiences, and helped to inform the design of this study. These contributions have been very meaningful and have made our work better.

If you have questions for Spark about our work, please feel welcome to send us a message at https://sparktx.com/medinfo/.

Sincerely,

Jacose Bell, Director of Patient Advocacy