In FSHD, despite all of the cells sharing the same genetic defect and similar epigenetic dysregulation of the pathogenic D4Z4 array and DUX4 gene, it is extremely rare to find DUX4-positive cells in muscle biopsies from patients, and even cultured FSHD myotubes show only ~1/1000 DUX4-positive myonuclei (Jones et al., 2012; Rickard et al., 2015; Tassin et al., 2013). This evidence concerns the gene DUX4 and facioscapulohumeral muscular dystrophy.