DUX4 and facioscapulohumeral muscular dystrophy: Firstly, in view of the convincing genetic data linking FSHD to the DUX4-fl RNA isoform [6-8], why was DUX4-fl RNA expression in FSHD myoblasts and myotubes extraordinarily infrequent, e.g., ~ 1 in 1000 myoblast nuclei positive [8]?