Pax5, also known as BSAP (B-cell-specific transcription factor) is a 42 kDa protein belonging to the paired box transcription factor family. It is a developmental regulator that is important for B-cell lineage commitment and development. Human Pax5 is a 391 amino acid (aa) protein containing the paired DNA-binding domain at the N-terminal region. Several alternatively spliced isoforms with altered C-terminal regions and possessing different transactivation properties have been described. All isoforms share the N-terminal 235 aa with full-length Pax5. Human Pax5 shares 99%, 97%, and 94% aa sequence identity with mouse, bovine, and canine Pax5, respectively.