Peptidoglycan recognition proteins (PGRPs) are pattern recognition molecules that recognize bacteria and their unique cell wall component, peptidoglycan (PGN). Mammals have four PGRPs, designated in humans PGRP-S(24kDa), PGRPL( 60~65kDa), PGRP-Iα(38kDa), and PGRPIβ( 44kDa). Human PGRPs are differentially expressed in various tissues and organs: PGRP-S mainly in the bone marrow and Polymorphonuclear leukocyte (PMNs), PGRP-L mainly in the liver, and PGRP-Ia and PGRP-Ib mainly in the esophagus. The function of only one mammalian PGRP, PGRP-S, is currently known. Mouse PGRP-S is present in neutrophils and inhibits growth of Gram-positive bacteria and, therefore, may function as a neutrophil antibacterial protein.