The 21-gene recurrence score assay was developed to predict which patients with oestrogen-positive breast cancer can avoid adjuvant chemotherapy. Although reported as a continuous variable, the score is usually split into three categories: low risk (score <18), intermediate risk (18–30), and high risk (≥31). Three published studies1 have reported that the risk of metastatic relapse is below 10% at 10 years in patients with low recurrence score, node-negative breast cancer treated with adjuvant tamoxifen....