DUX4 is expressed during zygotic genome activation, before being epigenetically repressed in somatic tissues.15 Mis-expression of DUX4 is believed to underlie FSHD pathogenesis but is extremely difficult to detect in FSHD muscle, with protein found in ∼1/1000 FSHD myoblasts ex vivo. Here, DUX4 is linked to facioscapulohumeral muscular dystrophy.