Acute myeloid leukemia with MLL rearrangement and monocytic differentiation. Although there is minimal staining with CD14, flow cytometric studies demonstrate other features associated with monocytic differentiation and a butyrate esterase cytochemical stain is positive. (A) Bone marrow aspirate smear demonstrating abnormal cells with moderately abundant cytoplasm, a few cytoplasmic granules, and some irregularity in nuclear outlines. Wright Giemsa stain, magnification ×100. (B) Representative flow cytometric dot plots: CD45 versus side scatter demonstrates a small population of lymphoid cells indicated in red, and a population of interest highlighted in green with weak intensity CD45 and variable side (orthogonal) light scatter; CD14 versus CD13⁺33 demonstrates staining for the myeloid antigens CD13 and/or CD33 and minimal staining for CD14; CD13⁺33 versus CD34 demonstrates absence of staining for CD34; CD15 versus CD33 demonstrates relatively bright staining for CD33 and variable intensity staining for CD15; CD36 versus CD64 demonstrates staining for both CD36 and CD64; and CD13⁺33 versus CD56 demonstrates partial aberrant expression of CD56. (C) Butyrate esterase cytochemical stain demonstrating many positive cells; magnification ×100. Staining was inhibited with fluoride incubation (not shown). (D) Classical cytogenetic studies demonstrating 47,XX,+8,t(11,19)(q23;p13.3). Courtesy of the Pittsburgh Cytogenetics Laboratory, Magee-Womens Hospital, Pittsburgh, PA. (E) FISH studies demonstrating an MLL gene rearrangement. Hybridization with the LSI MLL dual color DNA probe demonstrates one cell (lower left) with one fusion signal (corresponding to the unrearranged chromosome 11 at band 11q23) and separate green and red signals corresponding to the split MLL gene, and one normal cell (top right) with 2 fusion signals. Courtesy of the Pittsburgh Cytogenetics Laboratory, Magee-Womens Hospital, Pittsburgh, PA. Images were acquired as in Figure 1.