In different experiments PSGL1-P-selectin interaction was blocked by pre-incubation of leukemia cells for 30 min with hydroxyurea (1.4 mM) prior to addition of PMPCalcein, or by incubation of PMPCalcein with anti-P-selectin antibody (5 μg in 100 μl) for 30 min before mixing with leukemia cells [20, 23]. Here, SELP is linked to leukemia.