SMCHD1 and Facioscapulohumeral dystrophy: Indeed, (i) DUX4 protein and mRNA are detected in both adult and fetal FSHD1 and FSHD2 muscle cells and biopsies [148,149,150], (ii) hypomethylation of the D4Z4 array is always observed in FSHD patients, (iii) individuals carrying a permissive chromosome 4 but lacking the hypomethylation of the D4Z4 array are asymptomatic carriers [151], and (iv) the expression of DUX4 accounts for the majority of the gene expression changes in FSHD skeletal muscles [152].