ERBB2 and neoplasm: Furthermore, novel anti-HER2 drugs, particularly antibody-drug conjugates (ADCs), have shown great therapeutic potential even in tumors with low HER2 expression, largely due to the unique “bystander effect” of ADCs, which allows the drug to kill not only target cells directly bound by the antibody but also penetrate and kill surrounding tumor cells with even lower or negative HER2 expression.