Since the approval of trastuzumab and the IHC assay for HER2 overexpression (HercepTestTM, Dako) in 1998 by the US Food and Drug Administration (FDA), a number of new targeted cancer drugs guided by a diagnostic assay, a companion diagnostic (CDx) test, has been approved and introduced in the clinic to the benefit of the patients (5). The gene discussed is ERBB2; the disease is cancer.