The monoclonal antibody MD113 reacts with mouse MD-1. MD-1 (22-25 kD) is an extracellular protein associated with the extracellular domain of RP105. The latter is a type 1 transmembrane protein with leucine-rich repeats. RP105 is similar to Drosophila Toll. MD-1 associated with RP105 (similar to TLR4/MD-2) appears to be an essential molecule that helps RP105 on B-cells to signal to LPS. The MD113 antibody is antagonistic in LPS-induced B-cell proliferation and B7.2 up-regulation.