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The FISH method is fluorescence in situ hybridization, a laboratory technique used to detect and locate a specific DNA sequence on a chromosome. A fluorescently tagged DNA probe binds its matching sequence, and a fluorescence microscope shows where it bound. “FISH technique” names the same thing. The variants in this guide (Fiber-FISH, Q-FISH, Flow-FISH) change the sample format or the readout, not the basic idea.
Definition and scope
The National Human Genome Research Institute (NHGRI) defines fluorescence in situ hybridization as a laboratory technique for detecting and locating a specific DNA sequence on a chromosome. The U.S. National Library of Medicine MeSH vocabulary describes it as a type of in situ hybridization in which target sequences are stained with fluorescent dye, so that their location and size can be determined by fluorescence microscopy. MeSH says the method can be used on metaphase spreads and on interphase nuclei.
That places FISH inside the larger in situ hybridization family. The ISH guide covers probe design, controls and readout across the family; this page stays on the fluorescent DNA-on-chromosome case and its named variants. MeSH also lists “FISH Technique”, “FISH Technic” and “Fluorescent in Situ Hybridization” as entry terms for the same concept.
How the FISH method is used in practice
NHGRI describes the setup in three points. The chromosomes are fixed to a glass slide. A DNA probe carrying a fluorescent tag, with a sequence matching the region of interest, binds to that matching sequence. A specialized fluorescence microscope then shows where on the chromosome the probe bound.
The fluorescent tag is a fluorophore, and the choice of dye sets the filters and light source the microscope needs. See the guides on fluorophores and on FITC for spectra and labeling.
FISH variants compared
Bitesize Bio’s survey of FISH techniques notes that the variants differ mainly in probe sequence and labeling. The table below keeps only what that article states for each variant, alongside the NHGRI description of the standard method.
| Variant | What it works on | What it reports | Stated limit |
|---|---|---|---|
| Standard FISH | Chromosomes fixed to a glass slide (NHGRI) | Where a specific DNA sequence sits on a chromosome | None stated by NHGRI |
| Fiber-FISH | Stretched DNA fibers | High-resolution mapping | Described as a skilled art that is mostly done in specialized labs |
| Q-FISH | Probes for quantitative signal, mainly telomere length; PNA probes labeled with Cy3 or FITC | Signal intensity | Labor intensive and not suited to high-throughput work |
| Flow-FISH | Cells in suspension read in a flow cytometer (a 1998 modification of Q-FISH) | Median fluorescence across a cell population | Cannot give the telomere length of a particular chromosome in an individual cell |
PNA, peptide nucleic acid, has no charged phosphate groups, which according to the same article makes PNA-DNA binding stronger than DNA-DNA or DNA-RNA binding. For the flow cytometry side of Flow-FISH, see the flow cytometry guide.
Where it matters for labs
NHGRI lists four uses: diagnosing genetic diseases, gene mapping, identifying chromosomal abnormalities, and comparing gene arrangements across related species. What a lab needs on the bench follows from the description above: fixed chromosome preparations on glass slides, a labeled DNA probe, and a fluorescence microscope. Variant choice then follows the question. Position of a sequence on a chromosome points to standard FISH; high-resolution mapping on stretched fibers points to Fiber-FISH; telomere signal intensity points to Q-FISH, or to Flow-FISH when a population median is enough.
If the question is where an RNA or protein is, or what a probe signal means across a tissue section, neighboring methods fit better: Northern blot for RNA size and abundance, and expansion microscopy for physical enlargement of a fixed specimen before imaging.
Common misunderstandings
FISH and ISH are not two rival methods
MeSH classes FISH as a type of in situ hybridization. ISH is the umbrella; FISH is the version read by fluorescence.
“Fish method” and “fish technique” are the same search
Both phrases point to FISH, the abbreviation, and not to anything involving fish. MeSH carries “FISH Technique” as an entry term for the same concept.
Flow-FISH does not give single-chromosome results
Because Flow-FISH reports median fluorescence across a population, Bitesize Bio notes it cannot give the telomere length of a particular chromosome in an individual cell.
A bound probe shows location, not function
NHGRI describes the output as the location of a DNA sequence on a chromosome. Reading anything beyond that, such as expression or activity, takes a different assay.
FAQ
What does FISH stand for?
Fluorescence in situ hybridization. NHGRI describes it as a laboratory technique used to detect and locate a specific DNA sequence on a chromosome.
Is the FISH technique the same as the FISH method?
Yes. MeSH lists “FISH Technique” and “Fluorescent in Situ Hybridization” as entry terms for the same concept.
What is FISH used for?
NHGRI lists diagnosing genetic diseases, gene mapping, identifying chromosomal abnormalities, and comparing the arrangement of genes across related species.
Can FISH be used on cells that are not dividing?
MeSH says the hybridization signal can be seen both in metaphase spreads and in interphase nuclei.
What is the difference between Q-FISH and Flow-FISH?
According to Bitesize Bio, Flow-FISH was first published in 1998 as a modification of Q-FISH and quantifies median fluorescence in a population of cells, while Q-FISH can measure telomere length on a specific chromosome in a single cell and Flow-FISH cannot.
Sources
- NHGRI Talking Glossary, “Fluorescence In Situ Hybridization”: genome.gov/genetics-glossary/Fluorescence-In-Situ-Hybridization (checked 2026-10-29)
- U.S. National Library of Medicine, MeSH descriptor D017404, In Situ Hybridization, Fluorescence: ncbi.nlm.nih.gov/mesh/68017404 (checked 2026-10-29)
- Bitesize Bio, “Catch of the Day: A Look into Different FISH Techniques”: bitesizebio.com/23613 (checked 2026-10-29)
