Flow cytometry makes possible the rapid examination of large numbers of cells for their physical properties and/or phenotypic
profiles. Cells in suspension are examined with the the flow cytometer, also called a fluorescence activated cell sorter or
FACS, are first labeled with a fluorescent antibody or other fluorescent probe. Cells may also be distinguished in the FACS
based on their physical properties such as size or granularity.

When investigators bring samples to the FACS, they are usually interested in either,
 

• Quantitation. Cells have usually been labelled with a fluorescent antibody or other probe and the investigator wants toknow the percentage represented by the fluorescence-positive cells.
• Sorting. In this case an investigator wants to isolate, i.e., purify, a sub-population of cells.
• Cloning. Cells are placed singly in individual wells of a 96-well microtiter plate.
• Analysis. Biochemical parameters can also be examined with the FACS. For example, modulation of intracellular Ca2+ can be assayed using dyes such as Indo 1 and Fluo 3.
• DNA. Cell cycle distribution, i.e., %G1, S, and G2/M, can be determined with dyes such as propidium iodide.
• Apoptosis. Cells undergoing programmed cell death can be quantitated.