Fluorescent in situ hybridization (FISH) is a technique in which single-stranded nucleic acids (usually DNA, but RNA may also be used) are permitted to interact so that complexes, or hybrids, are formed by molecules with sufficiently similar, complementary sequences. Through nucleic acid hybridization, the degree of sequence identity can be determined, and specific sequences can be detected and located on a given chromosome. It is a powerful technique for detecting RNA or DNA sequences in cells, tissues, and tumors. FISH provides a unique link among the studies of cell biology, cytogenetics, and molecular genetics.

The method is comprised of three basic steps: fixation of a specimen on a microscope slide, hybridization of labeled probe to homologous fragments of genomic DNA, and enzymatic detection of the tagged probe:target hybrids. While probe sequences were initially detected with isotopic reagents, nonisotopic hybridization...