(A) Northern blot analysis of HB-EGF mRNA expression in different subtypes of ex vivo AML blasts. Human umbilical vein endothelial cells (HUVEC) were used as positive control. (B) 48-hour DT dose-sensitivity curve for 8 representative cases (2, 3, 4, 6, 7, 9, 10, 12 in A), as evaluated by the MTT method. Only HB-EGF mRNA-positive leukemic cells (cases 2, 3, 7, 9) were sensitive to DT, indicating membrane expression of the HB-EGF molecule.