In most mammalian species, serum amyloid A isoform 3 (SAA3) appears to be the predominant SAA isoform, expressed extrahepatically. However, human SAA3 gene expression has not been detected previously and, therefore, this gene was referred to as a pseudogene. We report for the first time the transcriptional expression of human SAA3. Human SAA3 gene expression was detected by RT-PCR after stimulation of mammary gland epithelial cells with either prolactin (PRL) or lipopolysaccharide (LPS). The full-length 655 bp cDNA sequence for this mammary-associated serum amyloid A3 (M-SAA3) was obtained using 5' and 3' rapid amplification of cDNA ends (RACE). The human M-SAA3 transcript would conceptually translate into a 42 residue mature protein, which is smaller than other mammalian SAA3 isoforms that are typically 104-113 amino acids in length. This study defines the cDNA sequence for human SAA3 and also demonstrates the upregulation of M-SAA3 expression in response to the lactational hormone PRL or to an acute phase stimulant such as LPS. (C) 2003 Elsevier Science (USA). All rights reserved.