Flow cytometry is a method used for cell counting, cell sorting, biomarker detection and protein engineering. It uses lasers to enable simultaneous multiparametric analysis of the physical and chemical characteristics of up to thousands of particles per second. Cells must be suspended in a stream of fluid and incubated with fluorescent-labelled antibodies which detect the expression of cell surface and intracellular molecules. The suspension is then passed by an electronic detection apparatus. The protocol for this technique is similar to but differs from that used for indirect flow cytometry, aka fluorescent-activated cell sorting (FACS).