Transcription factor that plays an essential role in commitment of lymphoid progenitors to the B-lymphocyte lineage (PubMed:9042861). Fulfills a dual role by repressing B-lineage inappropriate genes and simultaneously activating B-lineage-specific genes (PubMed:16546096). 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:9042861, PubMed:16546096). 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 (By similarity). Interacts with ETS1; this interaction alters PAX5 DNA-binding properties. Binds DNA as a monomer. Interacts with TBP; this interaction allows PAX5 to interact with the basal transcription machinery. Interacts with RB1. Interacts with TLE4 (By similarity). Interacts with DAXX (PubMed:11799127). Expressed in all B-lymphoid organs, in the embryonic midbrain 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. Mutant mice fail to produce small pre-B, B, and plasma cells and therefore lack any immunoglobulin in their serum owing to a complete arrest of B-cell development at an early stage.