Distinct CFU-S and radioprotective potentials of LSKCD34⁺flt3^– and LSKCD34⁺flt3⁺ ST-HSCs. (A) Lin^– BM cells expressing high levels of c-kit and Sca-1 (left panel) were investigated for expression of CD34 and flt3 (middle panel). Percentages indicate frequencies (mean of 8 independent experiments) within total BM of LSKCD34^–flt3^– (lower left quadrant), LSKCD34⁺flt3^– (lower right quadrant) and LSKCD34⁺flt3⁺ (upper right quadrant) cells. Boxes denote the sorting strategy used for each of the populations. Cells used in these studies were double or triple sorted, always resulting in 98% or higher purity (right panel). (B) Representative appearance of CFU-S_d11 colonies derived from LSKCD34⁺flt3^– and LSKCD34⁺flt3⁺ cells. Note the reduced number as well as average size of LSKCD34⁺flt3⁺-derived CFU-S colonies. Images were obtained with a Nikon Coolpix 995 digital camera (Nikon, Tokyo, Japan). (C) Lethally irradiated mice were given transplants with 500 LSKCD34⁺flt3^– (green) or LSKCD34⁺flt3⁺ (red) cells, and subsequently monitored daily for survival. Mice receiving no cells but injected with phosphate-buffered saline (PBS) served as negative controls (black). Results are from 2 experiments in which a total of 28 mice underwent transplantation in each group.