McBride and Ozer (1) were the first to show that purified metaphase chromosomes could act as vectors in transferring genetic information into mammalian cells. This technique, termed chromosome-mediated gene transfer (CMGT), involves the transfer of subchromosomal fragments from the cells of one species (the donor, usually human) to those of another (the recipient, usually a rodent) that have the ability to maintain them. CMGT thus fills a gap between the isolation of whole chromosomes, produced by cell fusion and microcell-mediated gene transfer, and the isolation of relatively short stretches of DNA by cloning or transfection methods.