MD-1 (also known as lymphocyte antigen 86) and MD-2 (also known as lymphocyte antigen 96 and ESOP-1) are secreted glycoproteins that associate with leucine-rich repeats of Toll-like receptor (TLR) family members. MD-1 and -2 aid TLR trafficking to the cell surface and mediate lipopolysaccharide (LPS) responsiveness. MD-1 and -2 share conserved cysteine residues and display 23% aa sequence similarity.