The present invention relates to quantitative molecular indicators that can guide clinical decisions in breast cancer, such as estrogen receptor (ESR1)-positive, lymph node-negative breast cancer. In particular, the invention concerns certain genes, the varied expression of which indicates the likelihood of recurrence of surgically resected breast cancer in patients who are not treated with a therapeutic agent in the adjuvant setting. In addition, the invention concerns the use of quantitative measurement of the expression of certain genes, including the ESR1 gene, that measures as a continuous variable, to determine (a) the likelihood of a beneficial response to the anti-estrogen therapeutic agent, such as tamoxifen; and (b) the potential magnitude of beneficial response to chemotheraphy.