The 21-gene Oncotype DX Breast Recurrence Score test is clinically validated as a prognostic tool that also uniquely predicts chemotherapy benefit in patients with node-negative or N+, HR+, HER2-negative breast cancer.16–19,26,33 We have summarized here the body of evidence supporting use of the 21-gene test in the HR+, HER2-negative, N+ setting. This evidence concerns the gene ERBB2 and breast carcinoma.