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:7791786, PubMed:8327485). 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). {ECO:0000250|UniProtKB:P41778, ECO:0000269|PubMed:12609849, ECO:0000269|PubMed:21746878, ECO:0000269|PubMed:7791786, ECO:0000269|PubMed:8327485, ECO:0000269|PubMed:9191052}. [Isoform PBX1b]: 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. . This is the function of ENSG00000185630 (PBX1, PBX homeobox 1).