DUX4 and facioscapulohumeral muscular dystrophy: A homologous D4Z4 array occurs at chromosome 10q, sharing ~99% sequence identity with its 4q counterpart but lacking the functional DUX4 polyadenylation signal that confers FSHD pathogenicity (van Geel et al. 2002; Clapp et al. 2007; Arends et al. 2025).