Fluorescent activated cell sorting (FACS) is a specialized type of flow cytometry\u00a0used for sorting and analyzing a heterogeneous mixture of cells into different subpopulations based on the specific light scattering and fluorescent characteristics\u00a0(from the specific labels) of each cell. The number of measurable parameters that\u00a0can be used by this technology to separate cell populations is immense \u2013 starting\u00a0from simple surface immunophenotyping to metabolic functions, cell cycle status,\u00a0redox state, and DNA content analysis to name a few.\nSince its inception, FACS has been used extensively in biomedical research and\u00a0clinical diagnostics and therapeutics. The most common usage of FACS is seen in:\n- Analysis of whole human blood for diagnosing diseases, immunophenotyping\n- Sorting different blood cell fractions for ex-vivo manipulations and\/or\u00a0transplantations\n- Immuno-phenotypic analysis of murine blood to identify transgenic\/knockout\u00a0animals\n- Sorting and analysis of a slew of cell lines for various biological assays\n- Characterization and isolation of rare cells types like adult stem cells and\u00a0cancer initiating cells Each human cell expresses hundreds of thousands of cell surface antigens that\u00a0specify their cell type, biological function, development stage, and much more. Cells\u00a0residing in different organs have characteristic cell surface antigens, and\u00a0determination of these cells using the specific fluorophore-conjugated antibodies can\u00a0be analyzed by flow cytometry. The following general protocols are recommended\u00a0for various common FACS staining procedures. Staining with unconjugated purified\u00a0antibody needs an additional step of staining with a fluorescent conjugated\u00a0secondary antibody (indirect immunostaining).