Oncotype DX(®) is a multigene assay that provides prognostic information in terms of 10-year distant recurrence and predicts the likelihood of adjuvant chemotherapy benefit in estrogen receptor positive breast cancer patients, based on the expression of a panel of 21 genes (16 cancer-related and five reference genes) from a tumor specimen (core biopsy or surgical resection). It has been validated using multiple prospectively designed studies of archived tumor specimens from well-controlled clinical studies. In different countries with varying therapeutic approaches, using Oncotype DX has consistently resulted in a significant reduction in the number of patients who are prescribed chemotherapy, and in addition, it can identify a smaller subset of patients who would benefit from chemotherapy among patients who would otherwise receive endocrine therapy alone.