This predefined subgroup analysis of the MONALEESA-2 trial demonstrates that postmenopausal women with de novo HR+, HER2− advanced breast cancer at diagnosis who received ribociclib in combination with letrozole had prolonged progression-free survival compared with those who received placebo plus letrozole, with an approximate 55% reduction in the risk of progression (hazard ratio 0.45, 95% CI 0.27–0.75). This evidence concerns the gene ERBB2 and breast cancer.