DUX4 and facioscapulohumeral muscular dystrophy: The short-term increase in mosaic bursting of DUX4 expression in skeletal muscles caused by TMX induction in the moderate ACTA1-MCM;FLExD FSHD-like mouse model was useful for pushing the system and enabling candidate DUX4-reponsive biomarker discovery; however, it is not very representative of the situation found in FSHD patients, who experience chronic, low levels of mosaic DUX4 expression in skeletal muscles, leading to accumulated muscle damage over their lifetimes.