Flourescent in-situ-hybridization test (FISH)

It is a cytogenetic technique that is used to detect/localize DNA sequneces on chromosomes. Flourescent in-situ Hybridization (FISH) is a big word, to understand it better, let’s break it down.

Flourescent – flourescent probes.  The flourescent probes are made from  fragments of DNA that were isolated, purified, and amplified for use in the Human Genome Project. These probes bind only to the DNA segment where there is a high degree of sequence similarity.  So in the case of CML, they will use probes specifically designed to detect the BCR ABL rearrangement that signals the Philadelphia Chromosome.

in-situ – it is a latin word that when translated literally means, “in position”.  However used in FISH it is referring to the chemical reaction and therefore means: “in the reaction mixture”

Hybridization – as in DNA hybridization, the process of joining two complementary strands of DNA.

So in a sample of your blood, the lab your hospital uses for this test can produce a picture showing exactly how many of your chromosomes are rearranged. At diangosis it is usually 100%, as you continue to follow your treatments, the ratio of Philadelphia Chromosomes drops.  Most patients usually achive FISH “Undetectable” by six months of treatments (with the newwer drugs).


It sounds complicated, but for the patient it requires a simple blood draw.

Ever wonder what the FISH pciture looks like?

Interphase of cells where the secondary colours shows the presence of BCR ABL, Philadelphia Chromosome

This picture is of interphase cells of a CML patient.  The secondary colours show the prescence of the the translocation which occurs because of BCR ABL, producing the Philadelphia Chromosome.

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