A Trial to Assess Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Preliminary Efficacy of SFL-0821 in Adults With FSHD

Inclusion Criteria: * Genetically confirmed diagnosis of FSHD 1 or 2 * FSHD Clinical Severity Score ≥1.5 to ≤4 at screening (5-point Ricci scale) * Ambulatory and able to walk 10 meters between ≥5 and ≤14 seconds (only assistive devices allowed are single cane or splints for ankle-foot orthoses) * Must have eligible lower extremity muscle for biopsy as determined from MRI by a central reader Exclusion Criteria: * History of any illness or any clinical condition that, in the opinion of the Investigator, might confound the results of the study or pose an additional risk in administering investigational product to the participant. * History of biopsy of the same muscle within 30 days of the Screening biopsy or planning to undergo any non-study muscle biopsies over the duration of the study * Treatment with an oligonucleotide, gene therapy, or other experimental therapies for FSHD.