DUX4 is embedded within the D4Z4 macrosatellite repeat [2], which is present in a tandem array at the terminus of chromosome 4q, and most FSHD-causing mutations are integral contractions of the D4Z4 array [3,4], although second site mutations that affect the chromatin state of the array also contribute [5,6]. The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.