SMCHD1 and facioscapulohumeral muscular dystrophy: However, not all FSHD2 patients can be explained by the lack of contraction of the D4Z4 repeats or mutations in the SMCHD1 gene, suggesting either that mutations may reside within the SMCHD1 non-coding region or that the cause of FSHD in these families could be linked to yet another FSHD locus.