What is being tested?
Her-2/neu is an oncogene. It codes for a receptor for a particular growth factor that causes cells to grow. Normal epithelial cells contain two copies of the Her-2/neu gene and produce low levels of the Her-2 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 (far too many copies are produced) and its protein is over-expressed (an abnormally large amount of the protein is produced). Tumours that have this over-expression tend to grow more aggressively and resist hormonal therapy and some chemotherapies, and patients generally have a poorer outcome.

There are two main ways to test Her-2/neu status in cancer tissue: 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.

How is the sample collected for testing?
A sample of breast cancer tissue is obtained by doing a fine needle aspiration, needle biopsy, or surgical biopsy. Her-2/neu protein sometimes is measured in a blood sample drawn from a vein in the arm. The amount of Her-2/neu protein present in serum is loosely associated with the amount of cancer present; however it will not be positive until the tumour is fairly big and is not widely used for determining Her-2/neu status.