DUX4 and facioscapulohumeral muscular dystrophy: There are many similar D4Z4 repeat arrays in the human genome, yet only the chromosome 4q35 and 10q26 D4Z4 arrays, which have >98% sequence identity between their respective D4Z4 RUs and are the only D4Z4 RUs to encode an intact DUX4 open reading frame (ORF) [5,28,52,53], are relevant to FSHD genetics and epigenetics [23].