DUX4 and Facioscapulohumeral dystrophy: FSHD has a complex etiology: insufficient epigenetic silencing caused by D4Z4 contraction in FSHD1 or heterozygous mutations in the chromatin modifier SMCHD1 in FSHD2 patients results in the inappropriate expression of the retrotransposed DUX4 gene in skeletal muscle [8], [9], [14], [25], [27], [34].