MEIS1 and leukemia: Finally, we performed gene expression studies in MLL-AF9 and MLL-AF6 transformed bone marrow cells and found that genes highly relevant to MLL leukemia (Hoxa5, Hoxa9, Hoxa10, Meis1, and Mef2C) were expressed at much lower levels in the ΔSET Ash1l versus WT Ash1l cells, (Fig. 1d).