Involved in B-lymphocyte antigen receptor signaling and germinal center formation. Expressed in embryonic blood vessel endothelial cells but not in the vasculature of adult organs. In adults, detected at high levels in hematopoietic and lymphoid organs. Up-regulated during germinal center formation. Mice show impaired antigen-dependent germinal center formation and B-cell antigen receptor (BCR) signaling. The number of mature B-cells is reduced.