Comparative genomic hybridization is a method of molecular cytogenetics based on DNA-hybridization. In this method, the whole DNA of the tissue under investigation serves as sonde in order to find by in situ-hybridization to normal metaphase chromosoms additions (insertions) or losses (deletions) of genetic material.
DNA of normal cells is used as reference and compared to the DNA tested. Reference-DNA and test-DNA are marked with different fluorescence-stains and mixed 1:1. This mixture serves as probe for the in situ-hybridization of metaphase chromosomes of lymphocytes of healthy donors. In binding to the corresponding DNA-sequences to metaphase chromosomes, the test-DNA and the reference-DNA compete. If the ratio of all chromosome segments of test- and reference-DNA is equal, there is a homogenousmixed staining. If, however, there is an increase of a DNA sequnce in one of the DNA , the staining is unbalanced.
For the diagnosis of hematological diseases such as myelodysplastic syndromes or for the estimate of the prognosis in chronic lymphocyti leukemia, CGH substitutes increasingly FISH-analysis. In contrast to FISH, CGN has the advantage to cover the whole genome and not only those lesions that are searched.