Normal CD34⁺ cell proliferation is more sensitive to elastase inhibition than CML CD34⁺ cells. 2 × 10⁴ CML or normal CD34⁺ cells/mL were cultured in 200 μL serum-free media in the presence of 10 ng/mL G-CSF, GM-CSF, and SCF with different concentrations of elastase. Proliferation was measured daily by tritiated thymidine uptake. (A) CML CD34⁺ cells (•, control; ○, 1 μg/mL elastase; □, 5 μg/mL elastase). (B) Normal CD34⁺ cells (•, control; ○, 1 μg/mL elastase; □,5 μg/mL elastase). (C-D) Percentage inhibition induced by, respectively, 1 or 5 μg/mL elastase (•, normal CD34⁺ cells; ○, CML CD34⁺ cells). Data represent mean ± SEM of 3 experiments.