Viability of normal human bone marrow cells (A and B) and CB CD34⁺ cells (C) in the presence or absence of PF4. (A) Effect of PF4 at different concentrations on the viability of normal bone marrow cells treated for 48 hours in culture. (B) Viability of cells incubated with or without PF4 (5 μg/mL) for 1 to 5 days. (C) Effect of PF4 on the viability of CB CD34⁺ cells with or without preincubation of PF4 (5 μg/mL) for 1 to 5 days. The arrow in (A) indicates a significant increase (P < .01) from this concentration point (1 μg/mL) or greater, and the arrow in (B) and (C) indicates that incubation of cells with PF4 from this day (day 1) up to day 5 resulted in a significant increase (P < .01) in viability.