V-erb-b2 erythroblastic leukemia viral oncogene homolog 3 (ErbB-3, HER3) is a member of the EGF receptor tyrosine kinase family including EGFR (HER1), Neu (ErbB-2, HER2), ErbB-3 (HER3), and ErbB-4 (HER4) that are frequently overexpressed in a variety of carcinomas. These family members form either homodimers or heterodimers upon ligand binding to mediate cell growth. ErbB-3 binds and is activated by neuregulins and NTAK. It forms a heterodimer with each of the other ErbB receptors. ErbB-3 is predominantly expressed in epithelial tissues and brain, and is overexpressed in a subset of human mammary tumors.