Although quantitative analysis of DUX4 protein expression by immunostaining was hindered by frequent high background staining in the myofibers and additional unspecific staining in interstitial cells, potentially immune cells, in MCM;FLExD-mature and -aged animals, overall, we observed that the number of recombined DUX4-expressing myonuclei increased over time, consistent with recent data that endogenous DUX4 expression is not immediately cytotoxic in FSHD myogenic cells (Chau et al., 2021). The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.