A technique in which a DNA probe, labelled with a fluorescent dye, base-pairs (hybridizes) with the complementary base sequence of a target nucleotide. It is used in genetic mapping for locating specific genes or cytogenetic abnormalities within a chromosome set. Another application is for locating particular messenger RNAs (mRNAs) within cells. If just a partial sequence of a gene is known, a DNA probe for the gene can be constructed. A preparation of the organism’s entire chromosome complement is then partially denatured to separate the DNA strands and incubated with the labelled probe. The probe binds to its complementary gene sequence, revealing the precise chromosomal location of the target gene. FISH is a valuable diagnostic tool for genetic diseases, and is used in research and clinical medicine to identify chromosomal aberrations such as deletions, duplications, or fusions. See also chromosome painting; spectral karyotyping.

https://www.genome.gov/10000206 FISH factsheet from the National Human Genome Research Institute