MPO and mastocytosis: Immunohistochemistry of bone marrow biopsy showed local proliferation of abnormal cells (30%) with CD34+, myeloperoxidase (MPO) (myeloid cells+), CD117+, CD2+, tryptase+, chymase+, and flow cytometry of bone marrow cells showed 47% abnormal cells expressing CD117, CD13, and CD25, but did not express CD34, CD2, and CD22, suggesting mastocytosis.