DUX4 and facioscapulohumeral muscular dystrophy: Presumably, the resulting ACTA1-MCM;FLExDUX4 mice would express low levels of leaky DUX4-fl, similar to an unaffected FSHD patient, and, upon limited TMX administration, would recombine the transgene in a fraction of myonuclei, dependent upon the amount of TMX, to produce a mosaic DUX4-fl expression pattern in the skeletal muscles.