We use the Danish subcohort of the large randomized Breast International Group (BIG) 1-98 trial of adjuvant tamoxifen versus letrozole (as monotherapy or sequentially) with the aim to investigate AIB1 as a prognostic and predictive biomarker in relation to adjuvant endocrine treatment in estrogen receptor-positive postmenopausal breast cancer. The gene discussed is ESR1; the disease is breast cancer.