A constitutively expressed serum amyloid a protein gene (SAA4) is closely linked to, and shares structural similarities with, an acute-phase serum amyloid a protein gene (SAA2)

The acute-phase reactant serum amyloid A (SAA) is a polymorphic apolipoprotein encoded by a family of highly homologous and closely linked genes: SAA1, SAA2, and SAA3. We have isolated a human genomic cosmid clone containing the gene encoding a fourth, constitutively expressed member of the human SAA superfamily, C-SAA, together with an SAA2*2 (SAA2β) gene. The gene encoding C-SAA shares the same 5' to 3' orientation as SAA2*2 and has the characteristic four-exon structure of the other members of the SAA superfamily. The exons of the gene encoding C-SAA share only limited sequence identity with those of SAA1, SAA2, and SAA3; they specify an mRNA, represented by the CS-1 cDNA reported previously by us, which is expressed at low levels (relative to the acute-phase SAAs) in normal and acute-phase liver. The gene encoding C-SAA is located 9 kb downstream of SAA2*2 and therefore occupies the locus that has been identified as containing the SAA4 gene.