Identification of tumor genomic alterations predictive of therapeutic benefit from targeted therapy has improved clinical outcomes across a wide range of malignant neoplasms. Examples include EGFR-mutant non–small cell lung cancer treated with epidermal growth factor receptor tyrosine kinase inhibitors, ERBB2 (formerly HER2/neu)-amplified breast cancer treated with human epidermal growth factor receptor 2–specific antibodies, and tumors with microsatellite instability treated with immune checkpoint blockade. This evidence concerns the gene ERBB2 and breast carcinoma.