Serum amyloid P component (SAP) belongs to a highly conserved superfamily of calcium-dependent ligand binding and lectin (carbohydrate binding) proteins, also called pentraxins, due to the pentameric structure of the proteins. The polypeptide subunits are 31 kDa in size under reducing conditions. The C-reactive protein (CRP) and SAP areshort pentraxins, which are produced in the liver in response to inflammatory mediators. Pentraxin 3 (PTX3), is the prototypic long pentraxin. Both SAP and CRP are evolutionary conserved in all vertebrates and also found in distant invertebrates such as the horseshoe crab (Limulus polyphemus). Human SAP and CRP share 51% residue sequence identity and 66% homology. Mouse SAP has shown to be a reliable marker of the acute phase. In mouse, SAP levels increase significantly 24 hours after challenge with lipopolysaccharide.In man, SAP does not become elevated during inflammation, whereas CRP is the prototypical acute phase reactant. CRP in mice has been isolated, but it has not been well documented as it occurs only at concentrations of nanograms per milliliter. SAP is highly resistant to proteolysis, especially when it forms complexes with calcium-dependent ligands. SAP avidly binds to macromolecular ligands, such as nucleosomal DNA, glycosaminoglycans and amyloid fibrils. Aggregated SAP can bind C1q and activate the classical complement pathway. SAP contributes significantly to the pathogenesis of amyloidosis. The mechanism of its participation is not yet known and may differ between species. The normal mouse serum level of SAP is similar between males and females, but is different among mouse strains. Endogenous SAP levels range from 20 µg/ml to 150 µg/ml. Mouse strains can be divided in high and low SAP-responder strains after stimulation.