A pre-specified primary aim of the study was to assess the degree to which the Recurrence Score would predict the risk of breast cancer-specific mortality among women with ER-positive, node-negative breast cancer treated with tamoxifen (that is, patients clinically similar to those included in the study conducted by Paik and coworkers [12]), either alone or in multivariate analyses with tumor size and tumor grade. This evidence concerns the gene ESR1 and breast cancer.