MD-1 and MD-2 are secretory glycoproteins that exist on the cell surface in complexes with transmembrane proteins. MD-1 is anchored by radioprotective 15 (RP15) which is a molecule containing leucine-rich repeats and is expressed on B cells, dendritic cells, and macrophages, while MD-2 is associated with TLR4. MD-1 is required for efficient RP15 cell surface expression and function. It is indicated that the RP15/MD1 complex, in conjunction with TLR4, mediates the innate immune response to LPS in B cells, and also plays a role in protecting against apoptosis, B-cell proliferation, etc. Mouse MD-1 cDNA encodes a 162 amino acid precursor protein with a putative 19 aa signal peptide and two potential N-linked glycosylation sites. It shares 4% and 66% amino acid sequence identity with chicken and human MD-1 respectively. MD-1 is mainly expressed in the spleen, and also detectable in the liver, brain, thymus, and kidney.