Groups of recipients were co-transplanted with 4 × 105 MLL-AF9 cells alone or 4 × 105 MLL-AF9 cells and increasing numbers of purified LSK cells, and then assessed for leukemia burden (via % GFP + MLL-AF9 cells) in the femoral bone marrow (A) and spleen (B) at 7 and at 12 days following transplantation. At 7 days there was a significant reduction in leukemic infiltration comparing 50,000 versus 10,000 LSK cells for both femurs (A) p=2.71×10−6 and spleens (B) p= 1.152×10−5. By day 12, all groups were grossly leukemic. As expected, MLL-AF9 cells administered alone rapidly constituted the entirety of the marrow, due to the ablation of endogenous hematopoiesis via the total body irradiation.