Immunohistochemical analysis of leukemic blasts from bone marrow biopsies. Bone marrow biopsies were taken before start of imatinib therapy. Immunohistochemistry was performed by applying the peroxidase/diaminobenzidine (DAB) method for anti–c-kit (A), antiphosphotyrosine c-kit (B), anti–PDGF-Rα (C), anti–PDGF-Rβ (D), and antiphospho-Akt (E). The specificity of positive phospho-Akt staining was controlled by the use of a specific blocking peptide (data not shown). Arrows indicate positive staining of AML blasts. Each figure shows a representative analysis of a total of 20. Original magnification, × 100.