Transcription factor which binds the DNA sequence 5'-TGATTGAT-3' as part of a heterodimer with HOX proteins such as HOXA1, HOXA5, HOXB7 and HOXB8 (PubMed:9191052). Binds to the DNA sequence 5'-TGATTGAC-3' in complex with a nuclear factor which is not a class I HOX protein (PubMed:9191052). Has also been shown to bind the DNA sequence 5'-ATCAATCAA-3' cooperatively with HOXA5, HOXB7, HOXB8, HOXC8 and HOXD4 (PubMed:8327485, PubMed:7791786). Acts as a transcriptional activator of PF4 in complex with MEIS1 (PubMed:12609849). Also activates transcription of SOX3 in complex with MEIS1 by binding to the 5'-TGATTGAC-3' consensus sequence (By similarity). In natural killer cells, binds to the NFIL3 promoter and acts as a transcriptional activator of NFIL3, promoting natural killer cell development (By similarity). Plays a role in the cAMP-dependent regulation of CYP17A1 gene expression via its cAMP-regulatory sequence (CRS1) (By similarity). Probably in complex with MEIS2, involved in transcriptional regulation by KLF4 (PubMed:21746878). Acts as a transcriptional activator of NKX2-5 and a transcriptional repressor of CDKN2B (By similarity). Together with NKX2-5, required for spleen development through a mechanism that involves CDKN2B repression (By similarity). As part of a PDX1:PBX1b:MEIS2B complex in pancreatic acinar cells, is involved in the transcriptional activation of the ELA1 enhancer; the complex binds to the enhancer B element and cooperates with the transcription factor 1 complex (PTF1) bound to the enhancer A element. Forms a heterodimer with MEIS1 which binds DNA (PubMed:19799567). The PBX1-MEIS1 heterodimer binds a cAMP-responsive sequence in CYP17 (By similarity). It also binds a consensus region in the SOX3 promoter (PubMed:19799567). PBX1 forms heterotrimers with MEIS1 and a number of HOX proteins including HOXA9, HOXD4, HOXD9 and HOXD10 (By similarity). Forms heterodimers with HOXA1, HOXA5, HOXB7 and HOXB8 which bind the 5'-TGATTGAT-3' consensus sequence (PubMed:9191052). Also forms heterodimers with HOXA5, HOXB7, HOXB8, HOXC8 and HOXD4 which bind the 5'-ATCAATCAA-3' consensus sequence (PubMed:7791786). Interacts with PBXIP1 (PubMed:10825160). Interacts with TLX1 (PubMed:19559479). Interacts with FOXC1 (PubMed:15684392). Interacts with MN1 (PubMed:31839203). Interacts with MEIS2 isoform 4, SP1, SP3 and KLF4. Part of a PDX1:PBX1b:MEIS2B complex; PBX1b recruits MEIS2B to the complex. Expressed in the kidney. Expressed in the endothelial cells of the glomeruli and interstitium (at protein level) (PubMed:28270404). Expressed in all tissues except in cells of the B and T lineage. Expressed strongly in kidney and brain (PubMed:28270404). The homeobox is required for PBX1 nuclear localization and for transcriptional activation of NFIL3. The disease is caused by variants affecting the gene represented in this entry. A chromosomal aberration involving PBX1 is a cause of pre-B-cell acute lymphoblastic leukemia (B-ALL). Translocation t(1;19)(q23;p13.3) with TCF3. TCF3-PBX1 transforms cells by constitutively activating transcription of genes regulated by PBX1 or by other members of the PBX protein family. TCF3-PBX1 binds the DNA sequence 5'-ATCAATCAA-3'. Belongs to the TALE/PBX homeobox family. Extended N-terminus.