Leukemic cells with conditional expression of MLL-ENL induce disease with shortened latency in secondary recipients. (A) Survival curves of primary and secondary recipients transplanted with immortalized or leukemic myeloid cells. Immortalized myeloid cells with conditional (ME4, ○) or constitutive (cME3, ◇) MLL-ENL expression were transplanted into primary recipient mice (n = 4 for each group). The resulting leukemic cells were transplanted into secondary recipients (● and ♦, n = 5 for each group). (B) Myeloid marker expression by leukemic cells. Dot plots show expression of CD45.2 and EGFP (left panel), and Gr1 and Mac1 (right panel), by leukemic ME4 and cME3 cells within the spleens of primary recipient mice. Numbers in the plots refer to the percentages of cells within each quadrant. (C) Immortalized cells and their leukemic derivatives show an identical pattern of retroviral integration. Southern blot analysis of genomic DNA isolated from immortalized cells ME4 and cME3 and their leukemic progeny ME4a, ME4b, and cME3a. Blots show 5′ (ME4, 4a and 4b) and 3′ (cME3, 3a) end-fragments produced by digestion of the integrated provirus and genomic DNA with BamHI.