DUX4 and facioscapulohumeral muscular dystrophy: FSHD is caused by the incomplete epigenetic repression of the double homeobox 4 (DUX4) retrogene, one copy of which is located in each unit of the D4Z4 macrosatellite repeat array on chromosome 4, most often either by D4Z4 repeat array contraction (FSHD1) or by mutations in the SMCHD1 gene (FSHD2), in conjunction with a permissive chromosome haplotype [17, 18].