Fluorescent In Situ Hybridization (FISH) has become a well-known method for genetic mapping and gene expression profiling. This technique exploits the use of fluorescent dye-labeled probes against a nucleotide sequence of DNA that is mapped or localized to a chromosome or intra-nuclear DNA to generate a fluorescence signal which in turn is visualized in situ by microscopy. The hybridization of the probe to the corresponding nucleotide sequence enables detection of the presence or absence of specific sequences, mapping of translocation break points, and determination of gene copy numbers.

As a diagnostic tool, FISH is widely used in clinical genetics, neuroscience, reproductive medicine, toxicology, microbial ecology, evolutionary biology, comparative genomics, cellular genomics and chromosome biology to check for genetic abnormalities associated with prenatal diagnosis, cancer, birth defects and mental retardation. It is possible to differentiate benign and malignant tumors in the same tissue using FISH. Disruption of regulatory elements in tissues is one of the main causes of tumor susceptibility which has been exploited to understand the aberrant gene functions. The traditional methods of morphological diagnosis are restricted in their use because tumors appear similar under a microscope. However, FISH overcomes this ambiguity by characterizing the genetic mutations as biomarkers.