Definition of FISH

FISH: Abbreviation for fluorescence in situ hybridization, a molecular technique used in chromosome studies.

FISH employs fluorescent (fluorescein tags) that glow under ultraviolet light to detect the hybridization (the combination) of molecular probes with specific chromosomes and specific chromosome regions. In FISH, the process vividly paints chromosomes or portions of chromosomes with fluorescent molecules.

This technique is useful for identifying chromosomal abnormalities and gene mapping. For example, a FISH probe to chromosome 21 permits one to "fish" for cells with trisomy 21, an extra chromosome 21, the cause of Down syndrome.