Deletions of the D4Z4 array to fewer than 10 repeat units or mutations in SMCHD1, a gene necessary for repeat-mediated epigenetic repression, result in decreased epigenetic repression of DUX4 in skeletal muscle, causing a human muscle disease, facioscapulohumeral muscular dystrophy (FSHD; OMIM #158900, #158901) [6], [9], [13]–[15]. The gene discussed is SMCHD1; the disease is facioscapulohumeral muscular dystrophy.