About 70% of the breast tumors diagnosed are estrogen receptor (ER)-positive and depend on estrogens and the interactions with their ER to grow and survive; their therapeutic treatment has a good clinical prognosis and effectiveness, but antitumoral treatment resistances and undesirable side effects (ovarian cysts, endometrial cancer, or blood clots) remain clinically challenging. This evidence concerns the gene ESR1 and endometrial cancer.