ESR1 and cancer: Breast cancer is the most frequently diagnosed type of cancer in women worldwide.1 A total of 60%‐70% of breast cancer cases are ERα positive, which can be well‐controlled by ERα selective antagonists such as tamoxifen.2 Tamoxifen has a similar structure as E2; however, it has an extra chain that interferes with the conformational change of ERα protein into the active form.3 Despite the effectiveness of tamoxifen treatment, a significant percentage of tumours with ERα expression develops endocrine resistance.4