B-cell-specific activator protein (BSAP) is a recently identified member of the Pax-gene family of transcription factors; in the lymphoid system, BSAP is produced only in B cells. Here, Markus Neurath, Eckhard Stüber and Warren Strober describe the molecular structure of BSAP and focus on the ability of this protein to regulate the expression of B-cell-specific genes. They propose that BSAP is a key protein of B cells and that it not only influence B-cell development but also influences the balance between B-cell proliferation and immunoglobulin secretion at later stages of B-cell differentiation.