Diagnosis and tests

FISH

The related article (“Cytogenetics: karyotype and FISH”) explains the blood count. This topic looks closely at fluorescence in situ hybridisation (FISH): how it works, when it is chosen and how the result is read.

Last updated: 14 October 2026~1 min read

Coloured dots on a dark background. — **Figure 1.**Red and green dots in a nucleus.

01Short answer

FISH is a test that finds a specific stretch of DNA in a cell’s nucleus using glowing fluorescent “probes”. A probe is a small piece of DNA that matches the gene region being searched for and carries a coloured label. In a normal cell, two dots are usually seen for each gene. In a translocation, two dots of different colours join (for example, red and green merge to look yellow); in a deletion there are fewer dots; with an extra chromosome there are more. FISH works on cells that are not dividing, and results come quickly, sometimes in a day or two. That is why it is used for urgent questions such as t(15;17) in APL or BCR::ABL1 in CML and ALL, and in CLL for a panel of del(17p), del(11q), +12 and del(13q).

02Simple explanation

Glowing probes find a specific gene.

Dots are counted.

Fast, with no need for dividing cells.

03Scientific explanation

Break-apart and fusion probes: break-apart probes show a gene breaking; dual-fusion probes show two genes joining.

Limitation: FISH looks only at the chosen target; unexpected changes need a karyotype or molecular tests.

04What happens in the body?

Red and green dots

In a normal nucleus, each probe shows two separate dots.

A yellow fusion

Two genes joined look like colours overlapping.

A merged red-green dot. — **Figure 2.**A yellow fusion signal.

The probe finds its target

A probe binds only to its matching piece of DNA.

A glowing probe on DNA. — **Figure 3.**The probe finds its target.

05How FISH is done

Four steps.

1FixingNuclei on a slide.

2ProbeA coloured probe is added.

3BindingIt joins the DNA.

4CountingDots are counted.

06Examples of FISH

Leukaemia	Change looked for
APL	t(15;17): PML::RARA
CML, Ph+ ALL	t(9;22): BCR::ABL1
CLL	del(17p), del(11q), +12, del(13q)
AML	KMT2A rearrangement, inv(16)
ALL	ETV6::RUNX1, KMT2A

07Key terms

FISH: Fluorescence in situ hybridisation.
Probe: A piece of DNA with a coloured label.
Signal: A glowing dot seen in the microscope.
Fusion: Two genes joined together.
Interphase: The stage of a cell that is not dividing.

08Key points

FISH looks for a specific change.
It works on non-dividing cells.
Results come quickly.
It is especially useful in APL and CML.
In CLL it is used as a panel.
It shows only what is looked for.

09Frequently asked questions

Can FISH be done on blood?

Yes, if there are enough leukaemia cells in the blood.

What does the percentage mean?

The share of cells carrying the change; your doctor will explain what it means.

Does FISH replace the karyotype?

No, they complement each other (“Karyotype”).

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

Can FISH be done on blood?
What does the percentage mean?
Does FISH replace the karyotype?

My questions

12Sources

National Cancer Institute (NCI). Definition: FISH
National Cancer Institute (NCI). Definition: cytogenetic analysis
National Cancer Institute (NCI). CLL treatment (PDQ), patient version
National Cancer Institute (NCI). Hairy Cell Leukemia Treatment (PDQ®) — Health Professional Version
American Cancer Society. Phases of Chronic Myeloid Leukemia
NHS. Symptoms of acute myeloid leukaemia
NHS. Symptoms of acute lymphoblastic leukaemia
NHS. Symptoms of chronic lymphocytic leukaemia
NHS. Symptoms of chronic myeloid leukaemia
American Cancer Society. Signs and Symptoms of Childhood Leukemia

Medical disclaimerThis information is for educational purposes only and does not replace medical advice.

Last updated: 14 October 2026← Back to the list

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