DUX4L9 and facioscapulohumeral muscular dystrophy: Intriguingly, a very small fraction (0.6%) of FSHD patients bear a genomic deletion encompassing the DUX4c locus, indicating that DUX4c does not cause FSHD (Lemmers et al., 2003; Deak et al., 2007), but DUX4c could instead modulate DUX4 function in FSHD.