Two different assays are clinically and commercially available for genomic characterization of tumors: the 21-gene OncotypeDx assay (Genome Health Inc, Redwood City, CA) for estrogen receptor (ER)-positive, early stage breast cancer [6,7] and the 70-gene Mammaprint (Agendia, Huntington Beach, CA) assay [4,5] for ER-positive and ER-negative early-stage, node-negative breast cancers. This evidence concerns the gene ESR1 and breast cancer.