SMCHD1 and facioscapulohumeral muscular dystrophy: FSHD is caused by DUX4 misexpression in skeletal muscle in response to germline deletions and rearrangements that contract the D4Z4 locus on chromosome 4 to have ten or fewer repeats (FSHD1), or by mutations in chromatin-modifying genes such as SMCHD1 and DNMT3B in combination with semi-short D4Z4 repeat lengths (FSHD2).