DUX4 and facioscapulohumeral muscular dystrophy: Thus, even though the pathogenic array is hypomethylated in all FSHD myocytes, CpG methylation levels are not significantly different in DUX4-expressing and DUX4 non-expressing myocyte populations suggesting that additional epigenetic signals are responsible for the sporadic DUX4 expression from pathogenic arrays in FSHD.