UNC1999, and not UNC2400, promotes differentiation, suppresses colony formation, and induces apoptosis of MLL-rearranged leukemia cells. (A) Representative light micrographs show Wright-Giemsa staining of MLL-AF9–transformed leukemic progenitors after treatment with the indicated concentration of UNC1999 for 8 days. Black bar, 10 µm. (B-C) Flow cytometry analysis of c-Kit and Gr-1 after treatment with 3 μM of the indicated compounds for 8 days. (D-E) Quantification of colony-forming units from MLL-AF9– (D) or MLL-ENL–transformed leukemia progenitors (E) after serial replating into the cytokine-rich, methylcellulose medium containing DMSO or 3 μM UNC2400 or UNC1999. Data are shown as the mean ± SD of experiments in duplicate. *P < .05; **P < .01; ***P < .001. (F) Light micrographs show typical morphology of the single-cell colonies derived from MLL-AF9–transformed leukemia progenitors following serial replating in the presence of DMSO or 3 μM UNC2400 or UNC1999. Black bar, 1 mm. (G) Percentage of live and apoptotic subpopulations of EOL-1 leukemia cells after the indicated compound treatments for 6 days. (H) Typical profiles of staining with PI and annexin V after treatment of EOL-1 cells with DMSO or the indicated concentration of UNC1999 for 6 days. PI, propidium iodide.