Murine CD79 is a disulfide-linked heterodimer composed of an a chain (CD79a) and a ß chain (CD79b) that associates non-covalently with membrane immunoglobulin (Ig) to form the B cell receptor (BCR) complex. Its expression is restricted to B lymphocytes first appearing on the surface at the pro-B cell stage prior to productive Ig gene rearrangements and remaining through all stages of B-cell differentiation prior to plasma cells. It has been proposed that the CD79 complex on pro-B cell surfaces may function to induce early B-cell differentiation.