ERBB3 (HER3) is a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases. While ERBB3 itself is lacking kinase activity, heterodimerization of the protein with other EGF receptors leads to the activation of cell proliferation and differentiation pathways such as PI3K/Akt. It is involved in the development of the nervous system and neural crest differentiation. ERBB3 has use as a biomarker in cancer, as amplification or overexpression has been reported in prostate, bladder, breast, devil facial tumor 1 (DFT1) and other cancers and is associated with worse prognosis in solid tumors. In immunohistochemistry of normal tissue, ERBB3 has high cytoplasmic and membranous positivity in the majority of tissues. References: Genes & Development. 1998. 12 (12): 1825–36. doi:10.1101/gad.12.12.1825, PMID: 9637684; Curr. Biol. 1998. 8 (1): R15–8, PMID: 9427620; Proc. Natl. Acad. Sci. U.S.A. 2003. 100 (15): 8933–8, PMID: 12853564; JNCI: Journal of the National Cancer Institute. 2013 Feb. 105 (4): 266–273, DOI: 10.1093/jnci/djs501; PLoS One. 2017 Jun 7;12(6):e0177919, PMID: 28591206; Br J Cancer. 1998 Nov;78(10):1385-90, PMID: 9823984