Flow cytometry enables accurate quantification of the fluorescence of objects in a population as they pass in front of a light beam. The objects may be fluorescently stained whole cells, nuclei, individual chromosomes, or even nucleotides. Cell cultures provide more of a problem for identification of the material, because of the lack of constant morphological features and instability of the karyotype. For results from flow cytometry to be of any value, it is essential that the identity of the species being worked with is known and verified. Since 1970, flow cytometry has been increasingly used in studies of animal, and particularly human, cytogenetics. All major hospitals dealing with immunology and cancer have flow cytometry facilities. In human genetics, pools of flow-sorted chromosomes are widely and routinely used for construction of chromosome-specific recombinant DNA libraries. The details of techniques for chromosome sorting are largely a function of the instrumentation used rather than the biology of the system.