The HER2 (erbB2/neu) receptor tyrosine kinase gene is amplified and overexpressed at the protein level in 20–30% of metastatic breast cancers. HER2 overexpression represents an example of oncogene addiction in many of these cancers, as HER2 blockade or kinase inhibition achieves durable responses in many patients with metastatic HER2-overexpressing breast cancer. The gene discussed is ERBB2; the disease is breast cancer.