In many situations in which a complex population is under investigation, there is frequently a need to isolate unique populations for further studies. The power of flow cytometry is that it is able to use multiparametric analysis to identify highly specific populations. Moreover, it is not just phenotypic characteristics identified by a specific antibody-antigen interaction that can be measured; it is also possible to measure the DNA content of cells (15), the RNA content (16), or even assess functional characteristics such as ion flux or pH (17) or altered cell states such as apoptosis and cell death (18).

Having identified a specific subpopulation using an analytical cytometer, why would it be important to sort or isolate particles? There is a large body of literature based on isolation of subpopulations by flow sorting that shows how useful the technology has become. It is possible to sort under aseptic conditions to isolate specific cell populations for expansion in culture (e.g., green fluorescent protein-tagged transfected cells) (19). Cells may be sorted to include in functional assays (20) or for transplantation into laboratory animals (21) or human patients (22). It is also possible to sort sperm for subsequent insemination to allow sex selection of offspring (23). Although flow sorters are used in the main to sort mammalian cells, it is more accurate to refer to particle sorting given that flow sorters have also been used to sort yeast (24,25), bacteria (26,27), and phyto-plankton (28). Indeed, whole cells are not always needed as it is also possible to sort subcellular organelles such as Golgi complex (29) or chromosomes (30). Flow sorting is the only practical way of isolating large numbers of specific chromosomes from humans, other primates (31), or plant species (32), and flow sorters proved invaluable during the human genome sequencing project (31) and more recently in the production of chromosome paints (33). In addition, as the newer scientific fields of genomics and proteomics have evolved, flow sorting has also become important in, for example, sorting large numbers of specific subsets of cells for microarray analysis (34). At the other end of the scale, single particles may also be sorted into individual wells of a plate for cloning (35) or for polymerase chain reaction (PCR) analysis (36). Therefore, the applications of flow sorting are wide-ranging and a flow sorter, or access to one, is an invaluable resource.

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