DUX4 and facioscapulohumeral muscular dystrophy: To study DUX4-mediated disease mechanisms in vivo, our laboratory recently developed an FSHD-like mouse model, the ACTA1-MCM; FLExDUX4 mouse, that exhibits mosaic DUX4 expression in skeletal muscle similar to the pattern found in FSHD patients [26, 27].