Oncotype DX Breast Recurrence Score® is a 21-gene prognostic and predictive assay indicated for use in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative, lymph node (LN)-negative or up to three LN-positive, early-stage breast cancer. The assay is used worldwide and is conducted at a CLIA-certified central laboratory in the USA. The 21-gene assay generates a Recurrence Score for each tumour sample, based on expression levels of 16 breast cancer-related genes, normalized to five reference genes. The Recurrence Score is a continuous variable that provides an individualized estimate of 9-year distant recurrence risk and the likelihood of adjuvant chemotherapy benefit. The 21-gene Recurrence Score assay is extensively validated in clinical studies, which are well supported by real-world registry studies. The assay informs treatment decisions and reduces adjuvant chemotherapy use in routine clinical practices, and is estimated to be cost-effective. The 21-gene Recurrence Score assay is included in all major international treatment guidelines. Currently, it is the only multigene assay validated for prediction of chemotherapy benefit, as well as for prognosis.