SMCHD1 and facioscapulohumeral muscular dystrophy: <h4>Background</h4>Facioscapulohumeral muscular dystrophy (FSHD) patients carrying borderline D4Z4 repeat units (DRUs) (8-10) represent a molecularly ambiguous group overlapping FSHD1 and FSHD2, characterized by pronounced phenotypic heterogeneity.