DUX4 and Facioscapulohumeral dystrophy: Detection of DUX4 mRNA in FSHD muscle cells for drug screening purposes is challenging due to low transcript levels that reflect the fact that DUX4 is expressed in only approximately 1 in 1000 FSHD myoblasts in culture [5], and that DUX4 mRNA is a target of nonsense-mediated decay [15] with a short half-life.