Breast Cancer Trials of Oral Everolimus-2 (BOLERO-2) study is a phase 3 randomized clinical trial that compares everolimus, an mTOR inhibitor, and exemestane versus exemestane and placebo in ER-positive postmenopausal patients with advanced breast cancer refractory to previous letrozole or anastrozole. This evidence concerns the gene ESR1 and breast carcinoma.