Interesting challenges ahead include uncovering the potential role of RANK/RANKL in estrogen-dependent proliferation in the endometrium, and determining whether the progestogenic effect, which limits proliferation in the endometrium, is in any way related to the RANK/RANKL system. The association between steroid hormones and cancer has been a primary research focus for many years. Reproductive hormones are of particular interest given the fact that several highly prevalent cancers in women, such as breast or endometrial cancer [[1]], are paradigmatic examples of endocrine-related cancer.