The disclosure provides methods for treating estrogen receptor positive (ER+) cancer in women with an effective amount of lasofoxifene, a pharmaceutically acceptable salt thereof, or a prodrug thereof. The disclosure also includes the detection of the Estrogen Receptor 1 (ESR1) gene mutations that lead to endocrine resistance and treatment of endocrine resistant ER+cancers.本發明提供用有效量之拉索昔芬(lasofoxifene)、其醫藥學上可接受之鹽或其前藥以治療女性的雌激素受體陽性(estrogen receptor positive;ER+)癌症之方法。本發明亦包括會導致內分泌阻抗性之雌激素受體1 (ESR1)基因突變之偵測及內分泌阻抗性ER+癌症之治療。