c-erbB-2 is a receptor tyrosine kinase. It exhibits extracellular domains with two cysteine-rich sequences, and a cytoplasmic tyrosine kinase domain flanked by large hydrophilic tails that carry several tyrosine autophosphorylation sites. Approximately 25% of primary breast and ovarian tumors were found to overexpress the protein. The c-erbB-2 protein is overexpressed in a variety of carcinomas especially those of breast and ovary.