Tpo counteracts apoptosis of CD34⁺CD38⁻ BM cells. Thirty thousand CD34⁺CD38⁻ BM cells were fixed directly after isolation (A), whereas 50,000 CD34⁺CD38⁻ BM cells were incubated in serum-depleted medium for 44 hours in the presence of Tpo (B) or in the absence of cytokines (C). A viable cell scatter gate was set to include greater than 90% of freshly isolated CD34⁺CD38⁻ BM cells (A). Cells falling within this gate were analyzed for the presence of fluorescein-dUTP labeling by flow cytometry as described in the Materials and Methods. Results are from one of three experiments, with similar results.