FISH micrograph of normal chromosomes. FISH (fluorescence in-situ hybridisation) is a technique that allows a gene (yellow) to be marked on a chromosome (red). FISH is mainly used to confirm the position of a gene, such as one that can cause an inherited disease, on the chromosome. It was also an important technique involved in finding all the genes in the human genome project. Genes are lengths of DNA (deoxyribonucleic acid) that control specific functions in an organism.