Fluorescent in situ hybridization (FISH) is an important 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. Automated FISH diagnostic systems have been approved by the US FDA under the medical device stream, a mark of the maturity and reliability of the technique.