IL21 and leukemia: Leukemia burden, as indicated by BM cellularity, numbers of MLL-AF9-GFP+Gr1+Cd11b+ leukemic cells in the spleen and BM, and leukemic blast frequency in the BM, was smaller in BL/6 AML mice compared to Il21−/− AML mice (Figures 1F and S1C–S1F).