Purpose This focused update addresses the use of MammaPrint (Agendia, Irvine, CA) to guide decisions on the use of adjuvant systemic therapy. Methods ASCO uses a signals approach to facilitate guideline updates. For this focused update, the publication of the phase III randomized MINDACT (Microarray in Node-Negative and 1 to 3 Positive Lymph Node Disease May Avoid Chemotherapy) study to evaluate the MammaPrint assay in 6,693 women with early-stage breast cancer provided a signal. An expert panel reviewed the results of the MINDACT study along with other published literature on the MammaPrint assay to assess for evidence of clinical utility. Recommendations If a patient has hormone receptor–positive, human epidermal growth factor receptor 2 (HER2)–negative, node-negative breast cancer, the MammaPrint assay may be used in those with high clinical risk to inform decisions on withholding adjuvant systemic chemotherapy due to its ability to identify a good-prognosis population with potentially limited chemotherapy benefit. Women in the low clinical risk category did not benefit from chemotherapy regardless of genomic MammaPrint risk group. Therefore, the MammaPrint assay does not have clinical utility in such patients. If a patient has hormone receptor–positive, HER2-negative, node-positive breast cancer, the MammaPrint assay may be used in patients with one to three positive nodes and a high clinical risk to inform decisions on withholding adjuvant systemic chemotherapy. However, such patients should be informed that a benefit from chemotherapy cannot be excluded, particularly in patients with greater than one involved lymph node. The clinician should not use the MammaPrint assay to guide decisions on adjuvant systemic therapy in patients with hormone receptor–positive, HER2-negative, node-positive breast cancer at low clinical risk, nor any patient with HER2-positive or triple-negative breast cancer, because of the lack of definitive data in these populations. Additional information can be found at www.asco.org/breast-cancer-guidelines and www.asco.org/guidelineswiki .