FISH micrograph of normal chromosomes. FISH (fluorescence in-situ hybridisation) is a technique that highlights a particular gene or chromosome for analysis. Here, it is being used on whole chromosomes to find any abnormalities, such as chromosomal pieces that have broken off one chromosome and attached themselves to another. Chromosomes contain genes, which are lengths of DNA (deoxyribonucleic acid) that control specific functions in an organism. Humans have 23 pairs of chromosomes. They appear X shaped and are made of a central zone called a centromere (pink), with long extensions called arms (blue). The chromosomes dyed green here are from the pairs numbered 2, 3 and 5.