However, as with FSHD1, FSHD2 also requires a disease-permissive 4A-type subtelomere allele distal to the D4Z4 array on at least one 4q chromosome [15], suggesting the expression of DUX4-fl is likely a key mechanism in both forms of FSHD. Here, SMCHD1 is linked to facioscapulohumeral muscular dystrophy.