Disappearance of CD25⁺ bone marrow mast cells following imatinib mesylate therapy. Multicolor flow cytometry results are shown for CD117⁺ high, side-scatter high bone marrow cells from a representative patient with the myeloproliferative variant of HES before (A) and at 1 month after (B) the initiation of therapy with imatinib mesylate. Cells were stained with FITC-conjugated mouse IgG1 control antibody (isotype control) and antihuman CD25 antibody (CD25). CD117⁺ high, side-scatter high bone marrow mast cells are shown in red and express CD25 prior to (Aii) but not following (Bii) imatinib mesylate therapy.