Meis1 acts as a transcriptional regulator of PAX6 and a transcriptional activator of PF4 in complex with PBX1 or PBX2. It is required for hematopoiesis, megakaryocyte lineage development and vascular patterning. It may function as a cofactor for HOXA7 and HOXA9 in the induction of myeloid leukemias.