High-resolution R-banding at the 1250-band level. II. Schematic representation and nomenclature of human RBG-banded chromosomes.

Detailed characterization of the RBG-banding pattern at the 1250-band level has been done after thymidine synchronization and block release with 5-bromo-2'-deoxyuridine (BrdU), which induces chromosome elongation and improves definition of chromosomal bands. Optimal conditions for the incorporation of BrdU and the use of the FPG (fluorochrome-photolysis-Giemsa) technique produced excellent band separation and band contrast even in highly elongated prophase chromosomes. Moreover, we did not observe lateral asymmetry in C-banded regions. The schematic representation of these elongated chromosomes in the 1250-band range per haploid set was prepared showing the relative position, the specific size, and the characteristic staining intensity for each band. To this idiogram was extended the International Standard Cytogenetic Nomenclature. This realistic idiogram should help in the preparation of R-banded prophase karyotypes and in the identification and localization of chromosomal rearrangements. Because differences exist between RBG and RHG bands, a brief comparative description of each RBG-banded chromosome is included. Moreover, a minute analysis of the banding pattern revealed that various parts of chromosomes contract differently. We also observed the presence of R-positive bands in heterochromatic regions of the short arms of the acrocentrics, and of chromosomes 1, 9, 16, and Y.