Pregnancy-specific glycoproteins (PSGs) are major placental proteins thought to be essential for the maintenance of gestation. Little is known about the regulation of expression of the 11 genes encoding these proteins. It was previously demonstrated that Kruppel-like factor 6 (KLF6) and specific-protein 1 (Sp1) bind to conserved sequence within the PSG-5 gene promoter. Informatics analysis revealed the presence of one potential binding site for Kruppel-like factor 4 (KLF4), in the PSG-5 promoter, suggesting a potential transcriptional regulator role for KLF4. Using gene promoter-reporter transfections and X-ChIP assays, we demonstrated that KLF4 is ail activator of the PSG-5 promoter by binding to a KLF consensus like binding which includes the Core Promoter Element region (-147/-140). Furthermore, we used previous data showing the binding of Sp1 transcription factor to a GT-box (-443/-437) and co-transfection assays with KLF4 and Sp1 to demonstrate the strong synergic activity of these two factors on the PSG-5 promoter. (c) 2006 Elsevier Inc. All rights reserved.