Somatic expression of DUX4-fl mRNA per se is not necessarily pathogenic since rare expression can be detected in some cultures of healthy myogenic cells and muscle biopsies, albeit at levels significantly lower than those found in similar tissues from FSHD-affected subjects, suggesting that the levels and/or timing of somatic DUX4 expression dictate disease [6–8]. Here, DUX4 is linked to facioscapulohumeral muscular dystrophy.