DNA from sorted chromosomes is suitable for construction of recombinant DNA libraries and for gene mapping. However, these studies require usable quantities of classified and purified chromosomal DNA. Conventional cell sorters (flow cytometers) have been widely used to study populations and DNA quantities of eukaryotic cells (see Chapter 12). Dual-beam, high-speed sorting based on cell sorters has been developed to facilitate purification of chromosomes by doublestaining with two different types of dyes, Hoechst 3325 (Ho) and chromomycin A3 (CA3) [1]. The Ho and CA3 contents of individual chromosomes are determined by measuring the fluorescence intensities, since the water droplets including the double-stained samples pass the focused UV and laser beams in the flow

chamber of the cell sorter. This method is based on measurement of fluorescence properties of the stained sample. However, this instrument is delicate, expensive, mechanically complex, and requires trained technicians for operation.