ESR1 and endometrial cancer: In endometrial cancer, estrogens drive proliferation via estrogen receptor α (ERα), which belongs to the superfamily of nuclear receptors and is encoded by ESR1. The presence of ERα is related to early-stage cancer, while a shift in the ratio between ERα and estrogen receptor β (ERβ) or loss of ERα are associated with shorter disease-free survival (Saegusa and Okayasu, 2000; Sakaguchi et al., 2002; Hu et al., 2005; Mylonas, 2010; Zannoni et al., 2013).