Theory of CGHComparative genomic hybridization (CGH) is a fairly new molecular cytogenetic technique that allows detection of DNA sequence copy number changes throughout the genome in a single hybridization. CGH is based on co-hybridization of differentially labeled tumor and normal DNA to human metaphase chromosomes. In a typical experiment: Thus, in a single hybridization it is possible to screen all chromosomal sites that may contain genes that are either deleted or amplified in cancer. SensitivityThe sensitivity of CGH in detecting gains and losses of DNA sequences is approximately 2-20 Mb. For example, a 10-fold amplification of a 200 kb region should be detectable under optimal hybridization conditions. Early CGH experiments were done using DNA isolated from freshly frozen tumor tissues. However, it has now been shown that DNA extracted from formalin-fixed paraffin-embedded archival tissue blocks can be used as well. Prior to CGH hybridization, DNA can be universally amplified using degenerate oligonucleotide-primed PCR (DOP-PCR), which allows the analysis of, for example, microdissected tumor samples. The latter technique is, however, more difficult and less reliable than CGH without PCR pre-amplification step. Applications of CGHCGH has been used to detect genetic aberrations in a variety of malignancies: