Constitutive expression of Fas on candidate human HSCs. Enriched CD34⁺ CB and BM cells were stained with Abs against CD34, CD38, and Fas and analyzed by flow cytometry. Viable cells expressing CD34 were gated and investigated for coexpression of CD38 and Fas. Panels show one representative experiment for CB and and one for BM cells. Percentages shown in quadrants represent mean values of at least 3 independent experiments.