Fluorescent in situ hybridization; 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).

With interphase FISH, probes are introduced directly to the interphase cell. Interphase FISH is often used for rapid detection of specific types of aneuploidy in fetal cells and for the detection of certain deletions, duplications, and other abnormalities in tumor cells. In contrast to metaphase FISH, interphase FISH does not permit visualization of the actual chromosomes; therefore, certain structural rearrangements or aneuploidy will not be detected.

With metaphase FISH, cells progress through the division process until metaphase, when chromosomes are condensed and can be individually distinguished. In contrast to interphase FISH, metaphase FISH permits visualization of the actual chromosomes as well as the general location of the abnormality on the chromosome.

Related term: aneuploidy