Introduction Fluorescence-activated cell sorter (FACS) separates a mixed population of cells after they have been labeled with fluorescent dyes which subsequently pass through a single-stream to then pass through a laser beam which excites and thus fluoresces the dye the scattered light created by the cell passing through this single-stream becomes detected by photomultiplier tubes this provides information about cell size and granularity expression of cell-surface proteins on individual cells within the cell sorter cells can be separated based on their binding with fluorescently labeled antibodies