ERBB2 and gastric cancer: The potency of the bystander effect might significantly increase clinical efficacy, particularly in the context of ADCs, as exemplified by the finding that trastuzumab deruxtecan (T-DX), an ADC that was proved to cause the bystander effect in preclinical settings, showed favorable results in a clinical trial of patients with gastric cancer even though trastuzumab emtansine (T-DM), which targets the same TAA as T-DX (HER2) but does not induce the bystander effect, showed no clinical effectiveness [91,92].