Normal CD34⁺ cell differentiation is blocked by elastase. 7.5 × 10⁴ normal or CML CD34⁺ cells/mL were cultured in 1 mL serum-free medium in the presence of 10 ng/mL G-CSF, GM-CSF, and SCF with different concentrations of elastase. After 10 days of culture nucleated cells were counted, harvested, and labeled with anti-CD66b and anti-CD34 antibodies. (A) Number of cells produced in response to increasing dose of elastase. Data represent mean ± SEM of triplicates (▪, normal CD34⁺ cells; □, CML CD34⁺ cells 1 μg/mL elastase). (B) Percentage inhibition induced by 1 or 5 μg/mL elastase. Data represent mean ± SEM of 4 different experiments (•, normal CD34⁺ cells; ○, CML CD34⁺ cells). ^*Indicates significant differences from normal values (^*P < .01). Phenotype of normal (C) or CML (D) CD34⁺ cells after 10 days of culture in the presence of 0, 1, or 5 μg/mL elastase (▦, CD34 cells; □, intermediate cells; ▪, CD66b cells). Data represent mean ± SEM of 3 experiments.