DUX4 and facioscapulohumeral muscular dystrophy: Interestingly, low somatic expression of DUX4-fl mRNA per se is not necessarily pathogenic since expression can be detected in some rare cultures of myogenic cells and muscle biopsies from healthy and asymptomatic FSHD subjects, albeit at levels significantly lower than in equivalent cells and tissues from FSHD-affected subjects [10, 18, 19].