Up-regulation of Fas expression on candidate human HSCs in response to in vitro cycling. CD34⁺ CB and BM cells were cultured in SF medium supplemented with SFT3 for 5 days in the absence or presence of TNF. Lin^- cells were gated and investigated for coexpression of CD34 and Fas. Flow profiles are from one representative experiment of CB and BM, respectively, whereas percentages in quadrants represent mean values from 3 independent experiments of CB and BM, respectively.