Acts as a transcriptional regulator of PAX6. Also acts as a transcriptional activator of PF4 in complex with PBX1 or PBX2. Required for hematopoiesis, megakaryocyte lineage development and vascular patterning. May function as a cofactor for HOXA7 and HOXA9 in the induction of myeloid leukemias. Interacts with the N-terminal region of PBX1 to form a heterodimer which binds DNA including a cAMP-responsive sequence in CYP17. Also forms heterodimers with PBX2. Forms heterotrimers with PBX1 or PBX2 and a number of HOX proteins including HOXA9, HOXD4 and HOXD9 where it acts as a non-DNA-binding partner. Also forms heterotrimers with PBX1 and HOX proteins including HOXD9 and HOXD10 where PBX1 is the non-DNA-binding partner. Heterodimer with DLX3. Heterodimer with HOXB13 (By similarity). Expressed at high levels in the lung with lower levels detected in the heart and brain. Expressed in pancreatic islets (beta-cells and non-beta-cells) (PubMed:21059917). Expressed at high levels in all stages of embryonic development analyzed (7 days to 17 days). Expression is coactivated by retroviral integration in BXH-2 murine myeloid leukemias. Meis1 serves as a site of viral integration in 15% of the tumors arising in BXH-2 mice that develop myeloid leukemia as a result of the expression of an ecotropic murine leukemia virus. Mice die between embryonic days 11.5 and 14.5, showing internal hemorrhage, liver hypoplasia and anemia. Belongs to the TALE/MEIS homeobox family.