Serum amyloid A (SAA) proteins comprise a family of small (12-14 kDa, 104-112 amino acid residues), differentially expressed proteins that are highly conserved among vertebrates. SAA proteins are involved in the acute phase responses; these are the immediate early host responses to inflammation. During the acute phase, circulating SAA levels are increased by 100-1000 fold, reaching concentrations of up to one milligram per milliliter. Liver is the major site of SAA synthesis, although extrahepatic expression has also been reported. Mouse SAA gene family maps to chromosome 7. In mouse, five SAA genes and four protein products have been identified: mouse SAA1, SAA2, SAA3 are the acute phase isoforms, SAA5 is constitutively expressed, and SAA4 is a pseudogene. SAAs belong to a category of acute phase proteins that also includes C-reactive protein (CRP), haptoglobin, ceruloplasmin, complement components C3 and C4, a1-acid glycoprotein, a1-proteinase inhibitor, and fibrinogen.