Receptor tyrosine-protein kinase ErbB3, also known as HER3 (human epidermal growth factor receptor 3), is a membrane bound protein that in humans is encoded by the ERBB3 gene. ErbB3 is a member of the epidermal growth factor receptor (EGFR/ERBB) family. ErbB3, like the other members of the EGFR family, consists of an extracellular ligand binding domain, a transmembrane domain, and an intracellular domain. ErbB3 has been shown to bind the ligands heregulin and NRG-2, but it can’t mediate the intracellular signal transduction through protein phosphorylation. However, its heterodimer with ErbB2 or other EGFR members responsible for tyrosine phosphorylation forms a receptor complex with high affinity, and initiates the related pathway which lead to cell proliferation or differentiation. ErbB3 has been shown to implicate in numerous cancers, including prostate, bladder, and breast tumors.