DUX4 and facioscapulohumeral muscular dystrophy: Importantly, we confirmed by RT-PCR that the disease-associated RNA isoform of DUX4, DUX4-fl RNA, is expressed at extraordinarily low levels in FSHD (but not detectable in control) myoblasts and myotubes despite the hundreds of genes dysregulated more than 2-fold in FSHD vs. control myoblasts and myotubes.