Apoptosis of normal and ET MKs relative to different TPO concentrations. CD34⁺ cells from 3 healthy subjects (•) and 3 patients with ET (▪) were cultured with various concentrations of TPO in serum-free conditions. After 7 (A) and 14 (B) days of liquid culture, cells were dual-labeled with annexin-V–FITC/CD41-PE. The percentages of apoptotic MKs from patients with ET were compared with those of healthy subjects after incubation with 0.1, 1, 10, and 100 ng/mL of TPO. Data are expressed as means (standard deviations were always within 6% of the means).