While testing of targeted therapy-relevant biomarkers is already standard for other cancer types like melanoma, lung, and colon cancer, the use of companion diagnostics for breast cancer was so far limited to the analysis of progesterone/estrogen receptors, human epidermal growth factor receptor 2 (HER2) and programmed cell death 1 ligand 1 (PD-L1) as well as to the analysis of germline alterations of BRCA1 and BRCA2 genes for inherited disease [2]. The gene discussed is ERBB2; the disease is cancer.