It is also worth mentioning that ADCs represent a promising treatment modality for HER2-positive cancers because of their intriguing mechanism that encompasses a potential bystander cytotoxic effect, since a significant portion of HER2-positive tumors have intra-tumor heterogeneity regarding the expression levels of HER2, and permeable cytotoxic payloads of ADCs can have cytotoxic effects on neighboring cells that do not have high HER2 expression [23,68,89]. This evidence concerns the gene ERBB2 and cancer.