Contracted D4Z4 arrays in FSHD1 (1–10 repeats on one 4q allele), or mutations in the chromatin modifier gene SMCHD1 in FSHD2, cause chromatin opening allowing for DUX4 transcription in FSHD muscle [6,15]. The gene discussed is SMCHD1; the disease is facioscapulohumeral muscular dystrophy.