A laboratory test that determines how many copies of a specific segment of DNA are present or absent in a cell.

Fluorescent in situ hybridization (FISH) is a laboratory technique that uses fluorescently-labelled pieces of DNA to detect specific genes, chromosome segments or chromosomes, in cells examined under a special microscope.

(synonym: FISH) A technique used to identify the presence of specific chromosomes or chromosomal regions through hybridization (attachment) of fluorescently-labeled DNA probes to denatured chromosomal DNA. Examination under fluorescent lighting detects the presence of the hybridized fluorescent signal (and hence presence of the chromosome material) or absence of the hybridized fluorescent signal (and hence absence of the chromosome material).