RP105 (CD180) was originally discovered as a mouse B cell surface molecule that transmits an activation signal. This signal leads to resistance against irradiation-induced apoptosis and massive B cell proliferation. RP105 is associated with another molecule, MD-1, which has an important role in the cell surface expression of RP105. MD-1, also known as lymphocyte antigen 68 and RP105 associated protein, associates with and regulates the cell surface expression of RP105. RP105/MD-1 constitutes an LPS-signaling complex on B cells and, like MD-2, enhances the LPS signaling via TLR4. MD-1 contains 162 amino acids and has a predicted 19 amino acid signal peptide and 2 N-glycosylation sites. MD-1 is highly expressed in B cells, monocytes and tonsil, and is localized on the surface of cells despite its lack of a transmembrane region.