PSF1 is an evolutionarily conserved DNA replication factor, which forms the GINS complex with PSF2, PSF3, and SLD5. The mouse PSF1 homolog has been identified from a stem cell-specific cDNA library. To investigate its transcriptional regulatory mechanisms during differentiation, we studied PSF1 mRNA expression in testis and characterized its promoter. No canonical TATA or CAAT boxes could be found in the PSF1 5'-flanking region, whereas several consensus AML1, GATA, and Sry putative binding sequences are predicted within 5 kb of the putative transcription start site. In addition, binding sites for oncoproteins such as Myb and Ets were also found in the promoter. In testis, various PSF1 gene transcription initiation sites are present and short transcripts encoding two novel isoforms, PSF1b and 1c, were found. However, spermatogonium stem cells specifically express transcripts for PSF1a. These data suggest that PSF1 is tightly regulated at the transcriptional level in stem cells.