Pathology dictionary

What is HER2?

HER2 is a protein that helps a cell grow. The HER2 protein sits on the outside of a cell and it can be switched on or off by proteins made by other nearby cells. Another name for HER2 is Her2/neu.

Different kinds of cells throughout the body make HER2 and use it to help them grow. Some cancers make extra amounts of HER2 receptor which allows them grow faster than normal cells.

Types of cancer that make extra HER2 include:

How do pathologists test for HER2?

Pathologists can perform tests to see how much HER2 receptor a group of cells are making. One type of test is called immunohistochemistry and it allows the pathologist to see the HER2 receptor on the outside of the cell. This test is given a score of 0 through 3.

Immunohistochemistry scores for HER2:

  • 0 and 1 – A score of 0 or 1 means the cells are not making extra HER2 receptor. These scores are called negative.
  • 2 – A score of 2 means the cells may be making extra HER2 receptor and another test called FISH (see below) will need to be performed to confirm the results. This score is also called equivocal.
  • 3 – A score of 3 means the cells are making extra HER2 receptor. This score is also called positive.

Another type of test is called fluorescence in situ hybridization (FISH). Instead of looking for HER2 on the outside of the cell, FISH test uses a probe which sticks to the part of the DNA in a cell that tells a cell how to make HER2. Tumour cells that make extra HER2 will also have more DNA instructions for HER2. Pathologists call this change a translocation.

FISH results for HER2:

  • Amplified – The cell has more DNA for HER2 and is making extra HER2 receptor.
  • Not amplified – The cell has the normal amount of DNA for HER2 is making normal amounts of HER2 receptor.

Why is this important? A patient with a tumour that makes extra HER2 will likely benefit from medications that block the action of HER2 such as trastuzumab.

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