Her-2/neu is an oncogene. It is a gene that codes for a protein that is a receptor for a particular growth factor that promotes cell growth. Normal epithelial cells found throughout the body contain two copies of the Her-2/neu gene and produce low levels of the Her-2/neu protein on the surface of their cells. In about 20-30% of invasive breast cancers (and some other cancers, such as ovarian and bladder cancer), the Her-2/neu gene is amplified and its protein is over-expressed. Tumors that have this over-expression tend to grow more aggressively and resist endocrine (anti-hormone) therapy and some chemotherapies. People with Her-2/neu positive breast cancers tend to have a poorer prognosis, but this tumor characteristic also makes them candidates to receive treatment specific for Her-2/neu-positive cancers.
To determine if a tumor is positive for Her-2/neu, a biopsy is taken and the sample of tumor is tested. There are two main ways to test Her-2/neu status: immunohistochemistry (IHC) and fluorescent in situ hybridization (FISH). IHC measures the amount of Her-2/neu protein present. FISH looks at the genetic level for actual gene amplification – the number of copies of the gene present. IHC is currently the most widely used initial testing method; however, if it is indeterminate or negative, then the FISH method is often done as a follow-up test.
A Her2/neu test blood test is also available. The amount of Her-2/neu protein present in the serum is loosely associated with the amount of Her-2/neu -positive cancer present. This test is not used for screening purposes and is not a substitute for tissue testing but may be ordered to help assess a person's prognosis and to monitor the effectiveness of treatment. After an initial diagnosis of metastatic breast cancer is made, this blood test may be performed and, if the initial level is greater than 15 ng/mL, then the test may be used to monitor treatment.