Fluorescence in situ hybridization. (FISH) is a technique in which a small fragment of DNA (The probe) from a particular gene or chromosome region is bound (hybridized) to chromosomes fixed in place (in situ) on slide. Since the probe is labeled with a fluorescent molecule, it is possible to see the location of the probe on the chromosome by using a fluorescence microscope. The technique is very useful for detecting deletions and duplications that are too small to be seen using standard chromosome analysis. It is also useful for rapid detection of missing and extra chromosomes and for detection of cancer specific chromosome rearrangement.

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