Mef2c is not necessary for MLL/ENL-induced transformation or maintenance in culture, but for homing and/or spread of the MLL/ENL leukemic cells in vivo. (A) Establishment of transformed cell lines with MLL/ENL is not impaired after deletion of the floxed Mef2c allele by CRE recombinase. Results show the mean colony number (± SD) of serially replated cultures seeded with 2 × 10⁴ cells in methylcellulose each week. The first plating was done immediately after MLL/ENL transduction, so that variation in the colony numbers reflects also infection efficiencies. Similar results were obtained for 2 independent M/E cell lines. (B) Inactivation of Mef2c in MLL/ENL-transformed Mef2c^fl/− cells did not alter their morphology (Pappenheim stain; 100×/1.3 oil) or gross immunophenotype, as assessed by FACS analysis. (C) Survival curves of NOD/scid mice injected intraperitoneally with 10⁶ M/E cells with indicated genotype and infected with a CRE vector. Mice were killed when large tumors in abdominal cavity were clearly visible. (D) Spleen weights of animals receiving M/E cells with the indicated genetic background. Vertical line indicates median value. P value was calculated by the Student t test.