Use of Biomarkers and Multigene Assays in Breast Cancer.

All patients with early breast cancer should be classified by subgroup through testing estrogen receptor, progesterone receptor, and HER2 status, with or without Ki-67 proliferation percentage. Genomic expression profiling aids clinical decision-making in most patients, because most are estrogen receptor-positive. The commercially available classifiers are prognostic for distant recurrence in node-negative and also node-positive patients. Patients at genomically low risk have excellent 5-year outcomes with endocrine therapy alone. Some assays also predict for benefit from adjuvant endocrine therapy and chemotherapy in node-negative and node-positive patients, but with varying levels of evidence. Genomic classifiers are also prognostic of late relapse and may predict benefit from extended adjuvant endocrine therapy.