DUX4 and facioscapulohumeral muscular dystrophy: Thus, the skeletal muscles from the adult pJ2-Cre:EGFP; R26NZG/+ mice exhibiting rare X-gal-positive nuclei, despite all the cells in the tissue containing the same D4Z4 transgene and reporter gene, mimic the rare DUX4 protein expression found in FSHD muscles.