We have investigated whether J_x recombination signal sequence (RS) binding protein (RBP-J_x) has any partial catalytic activities involved in the VDJ recombination reaction, such as cleavage, ligation, and bending of DNA. Murine RBP-J_x protein purified by J_x-RS affinity chromatography did not show DNA cleavage activities but contained a strong DNA ligase activity. To obtain a large amount of purified RBP-J_x- protein, recombinant RBP-J_x was synthesized in Escherichia coli as a fusion protein and also in silkworm cells. Although recombinant RBP-J_x- produced in silkworm cells could bind J_x-RS, it failed to show either ligase or DNA bending activity. Since the DNA affinity-purified RBP-J_x has the ligase activity, the RBP-J_x- protein may form a complex with a ligase in vivo. We have raised monoclonal antibodies against the RBP-J_x fusion protein which was synthesized in E. coli and unable to bind J_x-RS. Using the anti-RBP-J_x monoclonal antibody we have shown that the RBP-J_x protein is expressed ubiquitously in mammalian tissues. The ubiquitous expression of the RBP-J_x protein is consistent with the hypothesis that the RBP-J_x protein may have dual function [Furukawa et al. (1991) J. Biol. Chem. 266,23334– 23340].