Fluorescence in situ hybridization. In this technique, fluorescent molecules are used to label a DNA probe, which can then hybridize to a specific DNA sequence in a chromosome spread so that the site becomes visible through a microscope. FISH has been used to highlight the locations of genes, subchromosome regions, entire chromosomes, or specific DNA sequences. It has been used for mapping and the detection of genomic rearrangements, as well as studies on DNA replication.