If companion diagnostics (CDx) were people, they would be old enough to drink. The first CDx won FDA approval in 1998, when Dako Denmark (since acquired by Agilent Technologies) received authorization to market HercepTest. The immunohistochemistry (IHC) assay was designed to detect human epidermal growth factor receptor 2 (HER2) protein overexpression in breast tumors of patients who may benefit from Herceptin (trastuzumab), a targeted therapeutic for certain types of breast cancer marketed by Genentech (now a member of the Roche Group).