Multigene assays were developed to guide treatment in early-stage breast cancer and to evaluate the benefit of adjuvant chemotherapy. Patients with invasive breast carcinoma, either T1 or T2, N0 or N1 (one to three positive lymph nodes), with no invaded surgical margins (R0), positive for hormone receptors and negative for Her2 receptor derive benefit from these tests. The Food and Drug Administration (FDA) validated the use of the following tests: OncotypeDX (a 21 gene test), Mammaprint (70 ARN messenger genes), Predictor Analysis of Microarray 50-PAM50 (50 genes) also known as Prosigna and Endopredict (12 genes). These gene expression signatures have been increasingly used to evaluate the prognosis (such as Endopredict- that can predict outcomes more accurately than ki67 values in early-stage breast cancer) or the benefit of adjuvant chemotherapy (such as Oncotype DX). For the moment only two assays, the 70-gene signature (MammaPrint) and the 21-gene Recurrence Score (RS) assay (Oncotype DX), are supported by prospective randomized phase 3 trials. However, all multigene assays are important tools in the identification of low-risk, breast cancer patients, for whom chemotherapy could be avoided.