Very rare cases (2 families) of FSHD associated with a reduced number of D4Z4 repeats located in the subtelomere of 10q from de novo D4Z4 repeat exchange between chromosomes 4 and 10, indicate that D4Z4 units encoding DUX4 and a poly(A) signal are essential for pathology. Here, DUX4 is linked to facioscapulohumeral muscular dystrophy.