A tyrosine kinase receptor in the EGF-receptor family. ErbB2 does not bind ligand on its own, but acts as an amplifier in heterodimers with ErbB1, (EGFR), ErbB3, or ErbB4. Amplification of the ErbB2 gene and overexpression of ErbB2 protein is associated with breast cancer (~25â€¯% of cases), and anti-ErbB2 therapeutics (e.g. Herceptin, an anti-ErbB2 monoclonal antibody) are sometimes effective in treating these cases. Overexpression of ErbB2 has also been indicated as a factor in prostate cancer.