It is apparent that several of the major forms of ovarian cancer—namely HGSOC, LGSOC, endometrioid ovarian carcinoma and adult-type GCTs—show greater sensitivity to estrogen and can respond to strategies that either inhibit the production of estrogen (i.e., aromatase inhibitors) or that directly compete with its action at estrogen receptors (i.e., anti-estrogens such as tamoxifen or fulvestrant), which may have therapeutic value in selected groups of ovarian cancer patients. This evidence concerns the gene ESR1 and ovarian endometrioid carcinoma.