DUX4 and facioscapulohumeral muscular dystrophy: In vivo regulation of DUX4 within the context of a D4Z4 repeat array has previously been studied to some degree in the first published FSHD mouse models, the D4Z4-2.5 and D4Z4-12.5 mice, which differentially contained some of the human DUX4 regulatory elements (Krom et al., 2013).