Plays a role in B-cell proliferation and differentiation. Homodimer; disulfide-linked. Associates with CD5. Interacts (tyrosine phosphorylated) with PTPN6/SHP-1. Pre-B-cells and B-cells but not terminally differentiated plasma cells. Phosphorylated upon engagement of the B-cell receptor, probably by LYN or SYK. Phosphorylation at Tyr-7 is important for interaction with PTPN6/SHP-1. CD72