Transcription factor involved in B-cell differentiation. Binds a VH promoter proximal site necessary for induced mu-heavy-chain transcription. Binds the minor groove of a restricted ATC sequence that is sufficient for nuclear matrix association. This sequence motif is present in matrix-associating regions (MARS) proximal to the promoter and flanking E mu. Activates E mu-driven transcription by binding these sites. May be involved in the control of cell cycle progression by the RB1/E2F1 pathway. Homodimer. Heterodimer with ARID3B. Interacts with E2F1 (By similarity). Interacts with GTF2I and BTK. Shuttles between nucleus and cytoplasm. B-cell specific in the adult. Expressed in B-cell progenitors, down-regulated in the immature B-cell stage, and is up-regulated again at later stages of B-lymphocyte differentiation. Expressed in lymphocytes from fetal liver. Expressed in fetal thymus and brain.