ERBB2 and gastric cancer: In comparably designed studies (e.g., Gatsby study), in which a biomarker (Her2/neu)—also determined on the tumor cells—was combined with an antibody–drug conjugate (trastuzumab emtansine) specifically directed against the biomarker, the very heterogeneous distribution of Her2/neu in gastric cancer was a possible relevant reason for the failure of this study [36].