In general it is postulated that dysregulated expression of the HOXA9 gene cluster, which is under tight control by MLL1 during normal hematopoiesis, together with upregulated expression of another MLL target gene, the HOXA9 cofactor MEIS1, plays an important role in the development of leukemia as a consequence of the MLL translocation [14–18]. Here, MEIS1 is linked to leukemia.