 Side populations were first identified in hematopoietic stem cells by Dr. Margaret Goodell.SPs have been identified in hepatocellular carcinomas 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%.