The hierarchy of transcriptional control in B-cell development has rec
ently been analyzed by targeted gene inactivation in the mouse. In thi
s manner, the paired box containing gene Pax-5, encoding the B cell sp
ecific transcription factor BSAP, has been shown to play a key role in
early B lymphopoiesis. Other experimental strategies have implicated
BSAP in the control of cell proliferation, isotype switching and trans
cription of the immunoglobulin heavy-chain gene at late stages of B-ce
ll differentiation.