Inhibition of colony formation by FLAG-CrmA, FLAG–caspase-8–DN, and FLAG–FADD-DN does not appear to be due to induction of cell death. Fetal liver cells from E14 C57BL/6 embryos were infected with retroviruses encoding GFP alone, FLAG-CrmA, FLAG–caspase-8–DN, FLAG–caspase-9–DN, FLAG–FADD-DN, or FLAG-cFas for 48 hours in the presence of IL-6, mSCF, TPO, and FlkL. (A) After 2 days, viability of transfected (GFP⁺) cells was assessed by PI staining and FACS analysis. As a control to document that induction of apoptosis can be detected using this procedure, cells were infected with retroviruses encoding wt FLAG-FADD (52% ± 9% viability) or FLAG–caspase-8 (46% ± 3% viability). Numbers in quadrants represent percentages of cells within each staining profile. (B) Summary of cell-viability data representing means ± SDs from 3 to 6 separate experiments for each of the expression vectors. *P < .05. (C) Cell-cycle distribution of transfected (GFP⁺) cells was determined by staining fixed and permeabilized cells with PI. Data represent means ± SDs from 3 to 6 separate experiments for each of the expression vectors.