Side population (SP) cells are highly enriched for stem cell activity and characterized by their ability to efflux the vital dye Hoechst 33342,because they express the ATP binding cassette (ABC)-dependent transporter ABCG2.SP cells can be selected from main population using flow cytometric analysis.Currently SP cells have been isolated from many tissues and organs.SP cells of different origins have some common characteristics.This article introduces the classifications,surface marker,and characteristics of SP cells.