ERBB3 and cancer: In all but one cell line (PEA2-cis), co-treatment with the bispecific and tetravalent IGF-1R/ErbB3 IgG istiratumab resulted in reduction in cancer cell proliferation compared to untreated cells in the presence of IGF-1 (Fig. 3A), whereas in all but one cell line (PEA1) a decrease in proliferation was noted compared to untreated cells in the presence of HRG (Fig. 3B).