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 (By similarity). Binds the DNA sequence 5'-TGATTGAC-3' in complex with a nuclear factor which is not a class I HOX protein (By similarity). Has also been shown to bind the DNA sequence 5'-ATCAATCAA-3' cooperatively with HOXA5, HOXB7, HOXB8, HOXC8 and HOXD4 (PubMed:7791786). Acts as a transcriptional activator of PF4 in complex with MEIS1 (By similarity). Also activates transcription of SOX3 in complex with MEIS1 by binding to the 5'-TGATTGAC-3' consensus sequence (PubMed:19799567). In natural killer cells, binds to the NFIL3 promoter and acts as a transcriptional activator of NFIL3, promoting natural killer cell development (PubMed:32190943). Plays a role in the cAMP-dependent regulation of CYP17A1 gene expression via its cAMP-regulatory sequence (CRS1) (PubMed:7913464). Probably in complex with MEIS2, is involved in transcriptional regulation by KLF4 (By similarity). Acts as a transcriptional activator of NKX2-5 and a transcriptional repressor of CDKN2B (PubMed:22560297). Together with NKX2-5, required for spleen development through a mechanism that involves CDKN2B repression (PubMed:22560297). 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. (Microbial infection) In complex with PREP1, binds to the 5'-TGATTGAC-3' consensus sequence in the U5 region of Moloney murine leukemia virus and promotes viral transcription. Forms a heterodimer with MEIS1 which binds DNA (PubMed:9525891, PubMed:19799567). The PBX1-MEIS1 heterodimer binds a cAMP-responsive sequence in CYP17 (PubMed:9525891). 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 (PubMed:9315626, PubMed:10082572, PubMed:10523646, PubMed:12409300). Forms heterodimers with HOXA1, HOXA5, HOXB7 and HOXB8 which bind the 5'-TGATTGAT-3' consensus sequence (By similarity). Also forms heterodimers with HOXA5, HOXB7, HOXB8, HOXC8 and HOXD4 which bind the 5'-ATCAATCAA-3' consensus sequence (PubMed:7791786). Interacts with PBXIP1 (By similarity). Interacts with TLX1 (By similarity). Interacts with FOXC1 (By similarity). Interacts with MN1 (By similarity). Interacts with MEIS2 isoform Meis2D, SP1, SP3 and KLF4. Part of a PDX1:PBX1b:MEIS2B complex; PBX1b recruits MEIS2B to the complex. Expressed constitutively in natural killer cell precursors in bone marrow. The homeobox is required for PBX1 nuclear localization and for transcriptional activation of NFIL3. Conditional knockout in hematopoietic cells leads to a reduction in the number of natural killer cell-committed progenitors in bone marrow, decreases the number of natural killer cells in bone marrow and spleen and reduces NFIL3 expression in bone marrow and splenic natural killer cells. Belongs to the TALE/PBX homeobox family.