SMCHD1 and facioscapulohumeral muscular dystrophy: FSHD is caused by pathogenic D4Z4 repeat contractions (1–10 units) on a permissive 4qA haplotype (FSHD1) or mutations in chromatin-modifying genes (SMCHD1, DNMT3B, LRIF1) with a 4qA allele (FSHD2) [55].