SMCHD1 and Facioscapulohumeral dystrophy: FSHD results from a contraction at 4q35 resulting in too few D4Z4 repeats for efficient repeat-mediated epigenetic repression (FSHD type 1, FSHD1) or from the presence of mutations in trans-acting chromatin factors necessary for epigenetic repression of the D4Z4 array (FSHD type 2, FSHD2) [10–12].