Thus, initial analysis of the skeletal muscles of phenotypic ACTA1-MCM;FLExDUX4 mice indicate they share three key pathological characteristics of FSHD muscle: mosaic DUX4-FL expression that leads to DUX4-FL target gene expression, mononuclear cell infiltration, and increased fibrosis [5, 14, 15, 21, 61, 66]. This evidence concerns the gene DUX4 and facioscapulohumeral muscular dystrophy.