In FSHD, this region undergoes epigenetic alterations (DNA hypomethylation) following either contraction of the D4Z4 repeat array (FSHD1: OMIM #158900; 95% of cases) or by mutation in a chromatin organizer such as the SMCHD1 [40] or DNMT3B [41] genes (FSHD2: OMIM #158901; digenic inheritance pattern). The gene discussed is DNMT3B; the disease is facioscapulohumeral muscular dystrophy.