DUX4 and facioscapulohumeral muscular dystrophy: As discussed earlier, the D4Z4 repeat in pJ2-Cre:EGFP mice mimics the situation in FSHD, where the 4q35 D4Z4 array is epigenetically dysregulated in all cells and cell types (de Greef et al., 2009; Jones et al., 2014); however, despite the loss of this repression, only a small fraction of FSHD skeletal muscle cells express DUX4 at any given time (Haynes et al., 2018; Jones et al., 2012).