DUX4 and facioscapulohumeral muscular dystrophy: When combined in cis with a permissive 4qA haplotype supplying a poly(A) signal, the stabilized ectopic DUX4 mRNA [11] can be translated to generate DUX4 protein, which is believed to be the key driver of FSHD pathogenesis [7, 16, 17].