DUX4 and facioscapulohumeral muscular dystrophy: DUX4 expression levels depend on D4Z4 repeat length and methylation status, and can differ between cell lines isolated from different FSHD patients (Jones et al., 2012; Homma et al., 2012), as well as between genetically identical cell lines isolated from the same mosaic FSHD patient (Krom et al., 2012).