The advent of chromosome banding techniques some 20 years ago (1 ,2 ) allowed the unequivocal identification of every chromosome in the human metaphase and provided a mapping scheme along each chromosome. Subsequently, a great deal of research has centered on preparing longer chromosomes with more bands visible. Chromosomes condense as they move through mitosis, and adjacent bands close up and appear to fuse. The earlier stages are longer with more bands recognized. It is not always possible to define the mitotic stage of a particular cell. International standards have been agreed for various numbers of bands in the haploid set. Thus we have 400-, 550-, and 850-band sets (3 ). Other workers report the use of even longer chromosomes (4 ,5 ). High-resolution banding has undoubted advantages in many fields. As well as allowing greater accuracy in traditional karyotype analysis, there are many reports of microdeletions and other abnormalities detected only on extended chromosomes (6 ). Likewise, in situ hybridization and gene localization techniques are taking advantage of the improved resolution.