DUX4 and facioscapulohumeral muscular dystrophy: The most common form of FSHD, FSHD1 (>95% of cases), is linked to the mono-allelic contraction of the D4Z4 macrosatellite repeat array on chromosome 4q from 11–100 units to 1–10 units, with each 3.3 kb repeat containing the open reading frame for the double-homeobox transcription factor DUX4 [2–4].