DUX4 and facioscapulohumeral muscular dystrophy: Both forms of FSHD are characterized by aberrant expression of the double homeoprotein 4 (DUX4) transcription factor in the presence of a permissive 4qA haplotype (presence of a polyadenylation consensus sequence distal to the last D4Z4 repeat that stabilizes the DUX4 transcript) [5].