ESR1 and invasive breast carcinoma: •Postmenopausal women with estrogen-receptor-positive early invasive breast cancer not at low risk (those in the Excellent Prognosis Group or Good Prognosis Group in the Nottingham Prognostic Index)•Postmenopausal women with estrogen-receptor-positive early invasive breast cancer not at low risk and who have been treated with tamoxifen for 2 to 3 years