A cell surface protein-tyrosine kinase receptor that is found to be overexpressed in a significant number of adenocarcinomas. It has extensive homology to and can heterodimerize with the EGF receptor (RECEPTOR, EPIDERMAL GROWTH FACTOR), the erbB-3 receptor ($RECEPTOR, ERBB-$3) and the erbB-4 receptor. Activation of the erbB-2 receptor occurs during heterodimer formation with a ligand-bound erbB receptor family members.