Comparative Genomic Hybidization. CGH is a fluorescent molecular cytogenetic technique that identifies chromosomal aberrations and maps these changes to metaphase chromosomes. CGH can be used to generate a map of DNA copy number changes in tumor genomes. CGH is based on quantitative two-color fluorescence in situ hybridization (FISH). DNA extracted from tumor cells is labeled in one color (e.g., green) and mixed in a 1:1 ratio with DNA from normal cells, which is labeled in a different color (e.g., red). The mixture is then applied to normal metaphase chromosomes. Portions of the genome that are equally represented in normal and tumor cells will appear orange, regions that are deleted in the tumor sample relative to the normal sample will appear red, and regions that are present in higher copy number in the tumor sample (because of amplification) will appear green. Special image analysis tools are necessary to quantitate the ratio of green-to-red fluorescence to determine whether a given region is more highly represented in the normal or in the tumor sample.