The MammaPrint assay (Agendia BV, The Netherlands) is the first fully commercialized microarray-based multigene assay designed to individualize treatment for patients with breast cancer. MammaPrint, the first assay to be cleared at the 510(k) level by the US FDA's new in vitro diagnostic multivariate index assay classification, is offered as a prognostic test for women under the age of 61 years with either estrogen receptor-positive or -negative, lymph node-negative breast cancer. Unlike the Oncotype DX assay (Genomic Health, CA, USA), this test requires freshly prepared tissues collected into an RNA preservative solution. The 70 genes that comprise the MammaPrint assay are focused primarily on proliferation with additional genes associated with invasion, metastasis, stromal integrity and angiogenesis. The Microarray In Node-negative Disease may Avoid Chemotherapy (MINDACT) trial, sponsored by the European Organization for Research and Treatment of Cancer, involves the assessment of patients in the adjuvant treatment setting by the standard clinicopathologic prognostic factors included on Adjuvant! Online and by the 70-gene MammaPrint assay. The following article will consider the basic biology, technology, ease of clinical use, level of clinical validation and potential clinical utility of this test.