Chromosome banding is used mainly to identify both normal and rearranged chromosomes, to define chromosome breakpoints, and to describe the specific location of DNA sequences on chromosomes. A nomenclature has been developed to standardize the identification of chromosomes and the naming of chromosome bands. The system currently in use is An International System for Human Cytogenetic Nomenclature, referred to as "ISCN 1995." It is the report of the standing committee on human cytogenetic nomenclature edited by Felix Mitelman. The report includes a chromosome band nomenclature, as well as standard idiograms, which are "diagrammatic representations of a karyotype, which may be based on measurements of the chromosomes" (ISCN 1995). The idiograms presented here, with the permission of S. Karger and Cytogenetics and Cell Genetics, are drawings of G-banded chromosomes with band numbers indicated. Heterochromatic regions, which contain classes of repetitive DNA and can show individual differences in size, are indicated by patterned areas.