Acts as a transcriptional regulator of PAX6. 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 (PubMed:26550823). Heterodimer with HOXB13 (PubMed:28473536). Expressed at low level in normal immunohepatopoietic tissues, including the fetal liver. Expressed in a subset of myeloid leukemia cell lines, with the highest expression seen in those with a megakaryocytic-erythroid phenotype. Also expressed at high levels in the cerebellum. Disease susceptibility may be associated with variants affecting the gene represented in this entry. Belongs to the TALE/MEIS homeobox family.