ERBB3 and neoplasm: The specificity of HPK for HER3 has since been confirmed through several co-validating assays including receptor binding and competitive inhibition on immobilized HER3 in vitro and on HER3+ vs. HER3-deficient tumor cell lines; through the demonstration of preferential systemic accumulation in high- vs. low-HER3 tumors when both are present in mice; and by showing that the HPK nanobiologics exhibit increased binding and efficacy on trastuzumab- and lapatinib-resistant tumor cells concomitant with an increase in HER3 expression [22,23].