MAPK3 and Menkes disease: Quadriceps were then harvested at four months of age, which showed that relative to Wt controls, Sgcd−/− and Sgcd−/−; Mapk3−/−; Mapk1f/f disease control mice again had an expansion of satellite cells that is typical of early MD pathogenesis, while Sgcd−/−; Mapk3−/−; Mapk1f/f-Pax7Cre-ER mice lacking Mapk1/3 in their satellite cells showed a near complete loss of these cells (Fig. 2l).