DUX4 and facioscapulohumeral muscular dystrophy: Therefore, the pJ2-Cre:EGFP transgenic mice, which contain a single D4Z4 RU as found in severe FSHD1, when crossed with R26NZG reporter mice, allowed us to visualize the activity of the DUX4 regulatory elements in vivo under FSHD-like epigenetic conditions, and should be a useful tool for studying factors and stimuli that affect regulation of DUX4 expression in FSHD.