Previous studies showed that DUX4c is upregulated in FSHD myocytes and that it may disturb myogenesis and facilitate DUX4 toxicity,38,39 although in one FSHD family a proximal deletion at D4Z4 including the DUX4c gene was identified and patients have been diagnosed with FSHD linked to chromosome 10q where no complete DUX4c gene resides, suggesting that DUX4c is dispensable for FSHD pathogenesis.38 This evidence concerns the gene DUX4L9 and facioscapulohumeral muscular dystrophy.