Several multigene markers that predict relapse more accurately than classical clinicopathologic features have been developed. The 21-gene assay was developed specifically for patients with estrogen receptor (ER)-positive breast cancer, and has been shown to predict distant recurrence more accurately that classical clinicopathologic features in patients with ER-positive breast cancer and negative axillary nodes treated with adjuvant tamoxifen; validation studies in this population led to its approval as a diagnostic test. In a similar population, it also may be used to assess the benefit of adding chemotherapy to hormonal therapy. Other validation studies indicate that it also predicts the risk of distant and local recurrence in other populations with ER-positive disease, including node-negative patients receiving no adjuvant therapy and patients with positive axillary nodes treated with doxorubicin-containing chemotherapy. The Trial Assigning Individualized Options for Treatment (TAILORx) is multicenter trial that integrates the 21-gene assay into the clinical decision-making process and is designed to refine the utility of the assay in clinical practice and to provide a resource for evaluating additional molecular markers as they are developed in the future.