Even in these circumstances, expression of DUX4 is both rare and sporadic, with DUX4 found in as few as 1 in 2000 FSHD myoblasts and 1 in 200 myonuclei in FSHD myotubes (Box 1) (Block et al., 2013; Jones et al., 2012; Rickard et al., 2015; Tassin et al., 2013; van den Heuvel et al., 2018). The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.