DUX4 and facioscapulohumeral muscular dystrophy: FSHD is caused by mis-expression in skeletal muscle of the double homeobox 4 (DUX4) gene from the D4Z4 macrosatellite repeat in the subtelomere of chromosome 4 (4q35), either due to D4Z4 repeat contractions to 1–10 units or due to mutations in D4Z4 chromatin modifiers5–8.