PBX1, also named as Pre-B-cell leukemia transcription factor 1, is a 430 amino acid protein, which belongs to the TALE/PBX homeobox family. PBX1 is expressed in all tissues except in cells of the B and T lineage. PBX1 acts as a transcriptional activator of PF4 in complex with MEIS1. PBX1 may have a role in steroidogenesis and, subsequently, sexual development and differentiation. Isoform PBX1b as part of a PDX1:PBX1b:MEIS2b complex in pancreatic acinar cells is involved in the transcriptional activation of the ELA1 enhancer.