Transcription factor that plays an essential role in commitment of lymphoid progenitors to the B-lymphocyte lineage (PubMed:10811620, PubMed:27181361). Fulfills a dual role by repressing B-lineage inappropriate genes and simultaneously activating B-lineage-specific genes (PubMed:10811620, PubMed:27181361). In turn, regulates cell adhesion and migration, induces V(H)-to-D(H)J(H) recombination, facilitates pre-B-cell receptor signaling and promotes development to the mature B-cell stage (PubMed:32612238). Repression of the cohesin-release factor WAPL causes global changes of the chromosomal architecture in pro-B cells to facilitate the generation of a diverse antibody repertoire (PubMed:32612238). (Microbial infection) Plays an essential role in the maintenance of Epstein-Barr virus genome copy number within the host cell by promoting EBNA1/oriP-dependent binding and transcription (PubMed:31941781). Participates also in the inhibition of lytic EBV reactivation by modulating viral BZLF1 activity (PubMed:23678172). Interacts with ETS1; this interaction alters PAX5 DNA-binding properties (PubMed:11779502). Binds DNA as a monomer (PubMed:11779502). Interacts with TBP; this interaction allows PAX5 to interact with the basal transcription machinery (PubMed:10197586). Interacts with RB1 (PubMed:10197586). Interacts with TLE4 (PubMed:10811620). Interacts with DAXX (By similarity). (Microbial infection) Interacts (via N-terminus) with Epstein-Barr virus protein BZLF1 (via C-terminus); this interaction inhibits BZLF1-mediated lytic viral reactivation (PubMed:23678172). Interacts also with EBNA1; this interaction promotes EBNA1-dependent transcription (PubMed:31941781). Expressed at early B-cell differentiation, in the developing CNS and in adult testis. O-glycosylated. Phosphorylated by SYK. This phosphorylation plays an important role in the abolition of BLIMP1 repression by PAX5 in order to trigger plasma cell differentiation. A chromosomal aberration involving PAX5 is a cause of acute lymphoblastic leukemia. Translocation t(9;18)(p13;q11.2) with ZNF521. Translocation t(9;3)(p13;p14.1) with FOXP1. Translocation t(9;12)(p13;p13) with ETV6. Disease susceptibility is associated with variants affecting the gene represented in this entry.