FLOW CYTOGENETICS Flow cytometric analyses of individual chromosomes were made possible by the development of techniques for isolating intact chromosomes (1) and by procedures for staining chromosomes with DNA-specific fluorescent dyes. The term flow cytogenetics (2) was coined in the early 1980s to describe the application of flow cytometry and sorting to chromosome classification and purification. Two publications that appeared in 1975 described highresolution measurements of single mammalian chromosomes in a flow cytometer (3,4) and illustrated the use of sorting for chromosome purification (3). Although singlecolor measurements were an exciting development and a portent of what was to come, the development of twocolor staining provided the resolution needed to separate most of the human chromosomes. The introduction of Hoechst and chromomycin staining of mammalian chromosomes in 1979 (5) provided the breakthrough that allowed flow cytogenetics to achieve the successes described below. The ability to distinguish among individual chromosome types provided the first opportunities to distinguish and purify them in quantities suitable for visual, chemical, and molecular analyses. Initial sorting studies allowed chromosome types to be associated with flow karyotype peaks and provided chromosomes in a form that could be used for multiple purposes.