Pathology dictionary


What is HER2?

HER2 is a protein that helps a cell grow. The protein sits on the outside of a cell and it can be switched on or off by proteins made by other nearby cells. Different kinds of cells throughout the body make this protein and use it to help them grow.

Another name for this protein is Her2/neu.

Cancers that make HER2

Some cancers make extra amounts of HER2 receptor which allows them grow faster than normal cells. A patient with a tumour that makes extra protein will likely benefit from medications that block the action of HER2 such as trastuzumab.

Cancers that make extra HER2 include:

How do pathologists test for HER2?

Pathologists can perform tests to see how much HER2 protein a group of cells are making. One type of test is called immunohistochemistry and it allows the pathologist to see the protein 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 protein. These scores are called negative.
  • 2 – A score of 2 means the cells may be making extra protein and another test called fluorescence in situ hybridization (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 protein. This score is also called positive.
Fluorescence in situ hybridization (FISH)

Pathologists can also determine how much HER2 a cell is making by performing a test called fluorescence in situ hybridization (FISH). Instead of looking for HER2 on the outside of the cell, FISH 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 than the normal amount of DNA and is making extra protein.
  • Not amplified – The cell has the normal amount of DNA and is making normal amounts of the protein.
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