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 chro-
mosome. Subsequently, a great deal of research has centered on prepar-
ing 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-resolu-
tion banding has undoubted advantages in many fields. As well as allow-
ing 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.