The mouse and human genes which encode the serum amyloid A (SAA) proteins have been extensively characterized. In the present study, the authors report the sequencing of the constitutively expressed mouse Saa5 gene, including approximately 2 kilobases of promoter sequence which extends into the sequence of the adjacent Saa4 pseudogene. The Saa5 gene was thus determined to have a 4 exon/3 intron structure, with a B2 repeat located approximately 450 base pairs upstream, and a microsatellite contained within intron 2. Analyses of Saa5 intronic and promoter sequences, by comparison with representative members of the mouse Saa and human SAA families, provided evidence which strongly suggests that mouse Saa5 and the constitutively expressed human SAA4 gene are evolutionary and functional homologues. Furthermore, putative transcription motifs were identified within the Saa5 and SAA4 promoters which may account, in part, for the constitutive expression and unique induction profiles of these genes.