This study is testing a new drug, EPI-321, for facioscapulohumeral muscular dystrophy (FSHD) in adults aged 18 to 75. FSHD is a disease that weakens muscles, especially in the face, shoulders, and arms. EPI-321 uses a type of gene therapy to target and potentially fix the cause of FSHD. Participants will receive a single dose through a vein and be monitored in the hospital. Follow-up visits for tests and checkups will continue for about 5 years. The study aims to find out if the drug is safe, how well it works, and the best dosage for future studies.

Key Points:

• A single dose of EPI-321 is given, followed by regular clinic visits for 5 years.
• The study checks how safe the drug is and its potential effects on muscle function.
• Participants must be adults with FSHD Type 1 and meet certain health criteria.

Participants should not have certain health issues, be pregnant, or need a wheelchair. The study is open-label, meaning both doctors and participants know what treatment is being given. If you qualify and are interested, consider the time commitment and health checks involved.