ERBB3 and cancer: The results demonstrated that the new binders recognized native HER3 on several cancer cell lines, blocked the natural ligand heregulin from binding to the receptor on the cell surface, decreased the phosphorylation of HER2 and HER3 as well as the downstream signalling proteins Erk and Akt, and finally, inhibited the heregulin-induced proliferation of two HER3-positive cancer cell lines.