Lasofoxifene (Fablyn®), a SERM with benefits on bone health and potential for breast cancer prevention, has been investigated in mouse models of endocrine therapy-resistant BC with ERα mutations, Y537S and D538G, which have low sensitivity to fulvestrant inhibition, demonstrating its potential as an effective therapy for women with advanced or metastatic ER+ BC expressing the most common constitutively active ERα mutations [56]. The gene discussed is ESR1; the disease is breast cancer.