SP^low cells are phenotypically a homogeneous population. SP^low cells as gated in the top left panel usually represented the lowest 10% of SP cells in a given sort, with a bone marrow frequency of approximately 0.005% to 0.007%. Cell-surface marker expression on more than 500 SP^low cells was analyzed, with the percentages shown in the relevant quadrants. A representative analysis, out of at least 3 performed for each marker, is exhibited.