SMCHD1 and facioscapulohumeral muscular dystrophy: This epigenetic modification can be achieved by two non-mutually exclusive genomic events: either deletion of D4Z4 units to leave 1–10 repeats on at least one chromosome 4 as occurs in FSHD1 (MIM 158900) (>95% of FSHD cases) (15–17), and/or mutations in the chromatin-modifying gene SMCHD1 (18), or more rarely DNMT3B (19), in FSHD2 (MIM 158901).