Reduced SIRP expression on leukemic blasts. Mononuclear BM or PB cells from AML and CML patients were immunolabeled with CD45-FITC and MoAb P3C4 (IgG2a), followed by PE-conjugated goat anti-mouse IgG2a-specific antiserum (filled histogram). Nonbinding IgG2a antibody was used as negative control (black line). The histograms show representative examples of SIRP expression gated on CD45^low leukemic blasts. AML samples were divided into 3 groups: no expression (▵median of 0 to 5), reduced expression (▵median of 10 to 150), and normal expression of SIRP (▵median over 150). The number of analyzed samples is given in brackets.