ESR1 and breast cancer: The phase III ELAINE-3 study is assessing lasofoxifene + abemaciclib versus fulvestrant + abemaciclib in metastatic subjects with an ESR1 mutation progressing during AI plus palbociclib or ribociclib (NCT05696626) [161] The SERM/SERD bazedoxifene showed clinical efficacy as an anticancer agent in HR+ endocrine-resistant breast cancer models, particularly in Y537S ESR1 mutation carriers [162].