ESR1 and breast cancer: However, in the mammary tumors harvested two weeks after the mice were ovariectomized, the wt ERα was decreased to a nearly undetectable level while the 61-kD MC20RP was markedly increased, leading to a dramatic increase in the ratio of the 61-kD MC20RP to the wt ERα (arrowhead vs arrow in lane 4 in Fig. 4B).