Here, we show that the FLExD mouse model, when crossed with the skeletal muscle-specific and tamoxifen (TMX)-inducible ACTA1-Mer-cre-Mer mouse [55], produces a bi-transgenic model with chronic low levels of mosaic DUX4-fl expression in skeletal muscle and reproducibly recapitulating many aspects of FSHD pathophysiology. The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.