Tests and scans
FISH stands for fluorescence in situ hybridisation. It is a test that looks for gene changes in cells. Genes are made of . They control everything the cell does, including when it grows and reproduces. FISH tests look for specific genes or parts of genes.
Changes in genes can make the cancer cell:
produce particular proteins
make far more of the protein than normal
stop making a particular protein
If a gene change occurs, the cell may produce too much of a protein or not enough. This can make the cancer cells grow and reproduce more than normal.
There are now cancer treatments that target specific proteins. It helps your doctor to know if your cancer has a particular change in its genetic make up. This can tell them whether a particular treatment is likely to work for you.
Breast cancer
Some breast cancers produce too much of the protein because of a change in a gene called HER2/neu. If doctors need to find out more about HER2 levels, they can arrange a FISH test. It can show if the breast cancer is HER2 negative, HER2 positive or HER2 low.
Your doctor will work out the best treatment for you depending on the results of the test.
FISH testing is done by a pathologist. A pathologist is an expert who looks at and identifies cells.
To do a FISH test the pathologist needs some blood or tissue from your cancer. This can be from a , a blood sample or from tissue when you had surgery to remove your cancer.
In the laboratory, the pathologist attaches a dye and ultraviolet light to find and count the gene changes.
FISH tests are used in several cancers including:
breast cancer
chronic lymphocytic leukaemia (CLL)
acute lymphoblastic leukaemia (ALL)
acute myeloid leukaemia (AML)
chronic myeloid leukaemia (CML)
myeloma
Ewing sarcoma
lymphoma
lung cancer
melanoma skin cancer
bladder cancer
Go to our A - Z list to find out more about these cancer types
It's also used for non cancerous conditions such as Prade Willi syndrome and specific infections.
Doctors and researchers are finding out more about genetic changes that affect how cancer grows. FISH tests are an important part of finding more specific information about different cancer types. They can help doctors decide what treatment you need and to check how well treatment is working.
Last reviewed: 17 Jun 2025
Next review due: 17 Jun 2028
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