Fluorescence in situ hybridization is a cytogenetic technique that is used to detect and localize the presence or absence of specific DNA sequences on chromosomes.
FISH uses fluorescent probes that bind to only those parts of the chromosome with which they show a high degree of sequence complementarity.
Fluorescence microscopy can be used to find out where the fluorescent probe bound to the chromosomes.
Fluorescence in situ hybridization is used in genetic counselling, medicine, and species identification.
It can also be used to detect and localize specific mRNAs within tissue samples and thus identify organisms which otherwise are not identifiable on routine tests.
Further resources on FISH