The recurrence score based on the 21-gene breast-cancer assay was tested in 5018 patients for predicting a chemotherapy benefit in hormone-receptor-positive, HER2-negative, axillary lymph-node-positive breast cancer.Among premenopausal women with one to three positive lymph nodes and a recurrence score of 25 or lower, those who received chemoendocrine therapy had longer invasive disease-free survival and distant relapse-free survival than those who received endocrine-only therapy. This evidence concerns the gene ERBB2 and breast cancer.