DUX4L9 and facioscapulohumeral muscular dystrophy: Therefore, DUX4c might contribute to the FSHD phenotype by interfering with myoblast proliferation in one of two ways: (i) in most patients, DUX4c induction in myoblasts could perturb cell cycle progression or promote it in adjacent cells or (ii) in the few families in which the FSHD deletion has extended to loss of the DUX4c gene, there might not be sufficient protein to activate myoblast proliferation.