The low densities of EGFR and HER2 displayed on the NCI-H358 cancer cells used in this study strongly suggest that efficient tumor selectivity by dual-target avidity binding is not contingent upon overexpression of the two target antigens relative to healthy tissue, but rather primarily dependent on the intrinsic affinity of the two separate arms and the valence of the binding domains. Here, ERBB2 is linked to cancer.