HER2 is a type of protein called a receptor. It helps the cell respond to signals that tell it to grow and divide to create new cells. The protein sits on the outside of a cell and it can be switched on or off by proteins made by other nearby cells. HER2 is made by normal, healthy cells throughout the body. Another name for this protein is Her2/neu.
Some cancers make extra amounts of HER2 receptor which allows them to grow faster and divide more often than normal cells. Patients with tumours that produce extra HER2 will likely benefit from medications that block the action of HER2 such as trastuzumab.
Cancers that make extra HER2 include:
There are two tests that are commonly performed to measure the amount of HER2 in tumour cells. The first test is called immunohistochemistry and it allows your pathologist to see the HER2 protein on the surface of the cell. This test is given a score of 0 through 3.
HER2 immunohistochemistry score:
The second test that is used to measure HER2 is called fluorescence in situ hybridization (FISH). This test is usually only performed after a score of 2 on the immunohistochemistry test. Instead of looking for HER2 on the outside of the cell, FISH uses a probe that sticks to the HER2 gene inside the nucleus of the cell. Normal cells have 2 copies of the HER2 gene in the nucleus of the cell. The purpose of the HER FISH test is to identify tumour cells that have more copies of the HER2 gene which allows them to make more copies of the HER2 protein.
Tumour cells that make extra HER2 will also have more DNA instructions for HER2. Pathologists call this change a translocation.
HER2 FISH score: