ESR1 and ovarian cancer: The role of estrogens in the development of ovarian cancer is suggested by the following observations: (i) estrogen has a proliferative effect to ER-positive ovarian tumor cells; (ii) the application of estrogen-based hormone replacement therapy, which is frequently used among postmenopausal women in order to decrease the symptoms of age-related diseases, represents an important risk factor for the development of ovarian cancer; (iii) a total of 81% of ovarian tumors express ER; and (iv) ovarian cancer cells might have the capability to synthesize estrogen [20,21,22].