DUX4 and facioscapulohumeral muscular dystrophy: Confirming the importance of Dux4 expression as causal in FSHD, the onset of the muscle-wasting disease requires a patient to have a specific 4qA allele, which introduces a polyA signal downstream of the last copy of the D4Z4 array to permit the expression of stable DUX4 mRNA (15,16).