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Facioscapulohumeral dystrophy (FSHD) is one of the three most common muscular dystrophies in the Western world, however, its etiology remains only partially understood. Here, we provide evidence of constitutive DNA damage in in vitro cultured myoblasts isolated from FSHD patients and demonstrate oxidative DNA damage implication in the differentiation of these cells into phenotypically-aberrant myotubes. Double homeobox 4 (DUX4), the major actor in FSHD pathology induced DNA damage accumulation when overexpressed in normal human myoblasts, and RNAi-mediated DUX4 inhibition reduced the level of DNA damage in FSHD myoblasts. Addition of tempol, a powerful antioxidant, to the culture medium of proliferating DUX4-transfected myoblasts and FSHD myoblasts reduced the level of DNA damage, suggesting that DNA alterations are mainly due to oxidative stress. Antioxidant treatment during the myogenic differentiation of FSHD myoblasts significantly reduced morphological defects in myotube formation. We propose that the induction of DNA damage is a novel function of the DUX4 protein affecting myogenic differentiation of FSHD myoblasts.Constitutive DNA damage is present in cultured myoblasts from FSHD patients.DUX4 induces DNA damage when overexpressed in normal human myoblasts.DUX4 inhibition reduces the level of DNA damage in FSHD myoblasts.DNA damage is implicated in aberrant differentiation of FSHD myoblasts.Antioxidant treatment reduces morphological defects in FSHD myotube formation.DUX4-induced DNA damage affects myogenic differentiation of FSHD myoblasts.