ESR1 and cancer: It is well established that ERα is the major driver of the proliferative effects of estrogens in cancers of both breast and ovary, as well as in normal reproductive tissues, with ERβ serving largely as a brake for ERα-driven proliferation (Bossard et al. 2012; Madak-Erdogan et al. 2013; Paruthiyil et al. 2004; Strom et al. 2004).