The HER-2/neu oncogene encodes a transmembrane growth factor receptor protein that is overexpressed in a variety of cancer cells. The HER-2/neu oncoprotein has become a major target for the development of new cancer therapies. Studies show that monitoring serum HER-2/neu levels during metastatic breast cancer can provide a real-time assessment of a woman's HER-2/neu status and can provide important information for making therapeutic decisions.