ERBB2 and breast cancer: Different from BC, the incidence of heterogeneity in GC is much higher than that in BC (Rüschoff et al., 2012), and the heterogeneous expression of HER2 may affect the activity of trastuzumab since the nature of the non-cleavable linker does not allow bystander activity (i.e., the production of catabolic metabolites that are cytotoxic to neighboring tumor cells).