Schematic representation of the genetic mechanisms associated with facioscapulohumeral muscular dystrophy (FSHD). Normal D4Z4 alleles in chromosome 4 have 11 to 100 repeat units. In individuals with FSHD, one of the two D4Z4 alleles has a contraction mutation (1–10 repeat units) on a specific genetic background: a permissive haplotype 4A. However, contraction mutations of the D4Z4 locus on the non-permissive haplotype 4B are non-pathogenic (normal). Individuals lacking the D4Z4 contraction who have both mutations in SMCHD1 in chromosome 8 and a permissive haplotype in chromosome 4 develop an identical phenotype (FSHD2)