Although a copy of DUX4 resides within each RU of the D4Z4 array [37], only DUX4-fl transcribed from the distal-most 4q35 D4Z4 repeat is stably expressed in FSHD due to the presence of a polyadenylation signal (PAS) in a permissive 4A subtelomere-specific exon distal to the array, which is absent in 4B and other non-permissive subtelomeres [17]. The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.