RecruitingBiological interventionA First-in-human Study of EPI-321 in Facioscapulohumeral Muscular DystrophyThe goal of this clinical trial is to learn how safe and tolerable EPI-321 is and whether there may be early signs it is working in male or female adult (18 to 75 years) participants with facioscapulohumeral muscular dystrophy (FSHD) Type 1 condition. The main questions it aims to answer are: How safe is EPI-321 and how well can people handle it over time? How does EPI-321 interact with its target and does it show early signs of working? Participants will receive a single dose of EPI-321 through a vein while being closely watched in a hospital and visit the clinic regularly for tests and checkups for about 5 years after getting EPI-321.