May function as an activating coreceptor in B-cells. May function in B-cells activation and differentiation. Primarily expressed in secondary lymphoid tissues by mature subsets of B-cells. Detected in spleen, lymph node, heart, skeletal muscle, kidney, liver and placenta. Specifically expressed by mature B lineage cells with higher expression in naive versus memory B-cells (at protein level). Down-regulated in activated B-cells. Phosphorylated on tyrosines upon activation.