This could indicate that the myofibres in affected 3D-TESMs were atrophic, because DUX4 expression has been reported to cause atrophic myotubes in immortalized myoblasts.80 Alternatively, this could indicate that myofibres in affected 3D-TESMs were regenerating, which has been shown in FSHD muscle biopsies.81 In rats, regenerating skeletal muscle fibres produce less maximum force (∼10%) in comparison to non-regenerating muscle fibres,82 which might explain the lower absolute forces seen in affected 3D-TESMs compared with non-affected 3D-TESMs. The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.