Several lines of evidence indicate common mechanistic features between DUX4 and DUX4c due to the high similarity over most of the protein sequence (Bosnakovski et al., 2008a; Ansseau et al., 2009; Bosnakovski et al., 2009; Dmitriev et al., 2016; Knopp et al., 2016), suggesting that DUX4-DUX4c interplay may contribute to FSHD pathogenesis. Here, DUX4 is linked to facioscapulohumeral muscular dystrophy.