KMT2A and leukemia: Since MLL-AF9 and DNMT3A/FLT3ITD-driven leukemia may display the formation of extramedullary tumors, including in the spleen and liver32, we measured KLRG1+ST2+ Treg cells from other leukemic sites such as the spleen and liver and found similar phenotype as what was observed in the BM niche (Fig. 2A).