Mapping the position of genes in the cell nucleus sheds light on basic principles governing the genome. Here, a single gene called Pem (purple) has been localized using fluorescence in situ hybridization. DNA is stained blue; the cell cytoplasm is stained green. A gene is a locus (or region) of DNA that encodes a functional RNA or protein product, and is the molecular unit of heredity. Fluorescence in situ hybridization (FISH) is a cytogenetic technique that uses fluorescent probes that bind to only those parts of the chromosome with a high degree of sequence complementarity. It was developed by biomedical researchers in the early 1980s and is used to detect and localize the presence or absence of specific DNA sequences on chromosomes. Fluorescence microscopy can be used to find out where the fluorescent probe is bound to the chromosomes. FISH is often used for finding specific features in DNA for use in genetic counseling, medicine, and species identification. FISH can also be used to detect and localize specific RNA targets (mRNA, lncRNA and miRNA) in cells, circulating tumor cells, and tissue samples. In this context, it can help define the spatial-temporal patterns of gene expression within cells and tissues.