Multiple retrospective studies showed that Recurrence Scores using this 21 gene assay could not only estimate the risk of distant recurrence in ER+, node-negative breast cancers on NSABP B-14 and NSABP B-20 [48,49,50,51,52], however it could also predict breast cancers most likely to benefit from adjuvant chemotherapy [49]. This evidence concerns the gene ESR1 and breast cancer.