Conditional expression of CBFB-SMMHC in U937 leukemic cells blocks myeloid differentiation. U937 were treated with 0.5 μM ATRA for 5 days. CD11b expression was determined by FACS analysis (A-C) and morphology was examined by hematoxylin-eosin staining of cytospins at day 5 (D-F). Analysis was performed in U937T cells (A,D), in U937-tetoff-CBFB-SMMHC cells without induction of the CBFB-SMMHC fusion (B,E), and in U937-tetoff-CBFB-SMMHC cells following induction of CBFB-SMMHC protein (C,F). Gray lines indicate day 0; redlines, day 5 (A-C). Magnification (D-F) was × 40 with a Plan Fluor objective (Ph2 DLL; Nikon, Tokyo, Japan). Camera was a Nikon Digital Camera Dxm 12000. Acquisition software was from Lucia image (version 4; Prague, Czech Republic).