Side population

In flow cytometry, a side population (SP) is a sub-population of cells that is distinct from the main population on the basis of the markers employed. By definition, cells in a side population have distinguishing biological characteristics (for example, they may exhibit stem cell-like characteristics), but the exact nature of this distinction depends on the markers used in identifying the side population. Side populations have been identified in cancer and may be the cells that efflux chemotherapy drugs, accounting for the resistance of cancer to chemotherapy. Recent studies on testicular stem cells indicate that more than 40% of the SP (defined in this case as cells that show higher efflux of DNA-binding dye Hoechst 33342) was undifferentiated spermatogonia, while other differentiated fractions were represented by only 0.2%. The molecules involved in effluxing Hoechst 33342 are members of the ATP-binding cassette family, such as MDR1 (P-glycoprotein) and ABCG2.