c-kit is tyrosine phosphorylated in peripheral blood and bone marrow blasts from patients with AML. BM aspirates and peripheral blood sampling were performed at screening visits before start of imatinib therapy. MNCs were separated, and whole cell lysates were prepared as described in “Patients, materials, and methods.” Western blot analysis was performed by using an antiphosphotyrosine c-kit antibody. To control for equal loading, the blot was stripped and reprobed with anti–c-kit antibody. Shown are representative results from 3 patients (PN 9, 13, and 14). AML blasts obtained from PN 9 were incubated with and without SCF (100 ng/mL) for 15 minutes. Arrows indicate the position of the 160-kD isoform of c-kit.