HOXA9 and leukemia: Accordingly, we found that leukemic cells derived from co-cultures with Pml−/− MSCs (with both HoxA9–Meis1 and MLL/AF9 models) displayed overt functional disadvantages: leukemic cells showed a marked reduction in their capacity to form colonies when plated as single cells in a methyl-cellulose medium (Supplementary Fig. 4a) and a significantly reduced ability to generate leukemia when transplanted in mice (Fig. 3c and Supplementary Fig. 4b, c).