This approach to the treatment of cancer patients began in the early 1970s with the introduction of estrogen receptor (ER) testing and reached a major milestone with the approvals of the first combination of a drug, trastuzumab and a companion diagnostic test, the HercepTest for the treatment of patients with ER-positive and HER2-positive breast cancer [34, 35]. The gene discussed is ERBB2; the disease is breast cancer.