DUX4 and facioscapulohumeral muscular dystrophy: The root cause of FSHD is misexpression of the double homeobox 4 (DUX4) retrogene in skeletal muscle (Gabriëls et al., 1999; Kowaljow et al., 2007; Lemmers et al., 2010; Snider et al., 2010; Tassin et al., 2013).