FSHD results from the loss of this epigenetic repression, either due to contraction of the D4Z4 array (FSHD type 1, 95% of patients) or mutations in genes involved in maintaining repression via structural maintenance of chromosomes, such as SMCHD1 (FSHD type 2, 5% of patients) [38],[39]. This evidence concerns the gene SMCHD1 and facioscapulohumeral muscular dystrophy.