Characterization of S- and L-cell subpopulations including previous data22 and in this study. For each parameter, intensity or level of expression is described: − none, + low, ++ medium, +++ high, ++++ very high; () indicates percentage of cells expressing the specific phenotypic marker. For measures of proliferation, proportion of S-phase and double-labeled cells are expressed as percentages. Numbers and area of bone nodules reported per microscopic field. Osteoprogenitor frequencies were computed from limiting dilution analyses and are expressed as prevalence of progenitors in whole cell population that are capable of forming bone nodules. In criteria for self-renewal, sorting purity was calculated from the proportion of S⁻ cells that exhibited the same scatter characteristics as the initial sort. Collectively, the data show that S⁻ cells are enriched for stem cell markers as described in reference 2.