Detection of specific nucleic acids (DNA or RNA) by hybridization of fluorescence dye-conjugated probes. FISH can be used to identify specific chromosomal structures.
Also, FISH is being used in histological sections of tissues to determine the level of mRNA expression. In both settings, evaluation of hybridization signals is being done by fluorescence microscopy.
For chromosomal FISH, probes can be used that are specific for the centromere region of particular chromosomes, for genes, or for complete chromosomes. The DNA of both the applied probe and of the patient sample are denaturated, i.e., both DNA strands of the double helix are separated. During the following renaturation, the DNA probes attach to the complementary section of the patient DNA (hybridization). The DNA probes are either directly conjugated to a fluorescent dye or are analyzed using fluorescence conjugated antibodies. The respective chromosome structures therefore are assessable as fluorescence signals. For FISH on tissue sections, in principle a similar technology is employed.