Induction of apoptosis by different anti-CD99 mAbs. mPB CD34⁺ cells were incubated with 20 μg/mL 12E7 (▪), O662 (□), and hec2 (). The proportion of annexin V⁺ cells relative to the untreated control (arbitrarily 100%) was evaluated after 5 hours in the absence of cytokines and after 24 hours of culture in a medium supplemented with IL-3, IL-6 (10 ng/mL each), and SCF (100 ng/mL). Data are mean ± SEM of 3 experiments.