Given that the uncertainty of chemotherapy benefits remains in intermediate-risk patients, especially with the RS of 26–30 in hormone receptor-positive, HER2-negative, and node-negative breast cancers, we investigated whether adjuvant chemotherapy was associated with breast cancer-specific mortality and overall mortality in breast cancer patients with the RS 18–30, particularly among those aged ≤ 70 years with the RS 26–30. This evidence concerns the gene ERBB2 and breast carcinoma.