Nanog gene plays a key role in maintaining pluripotency of ES cells and early embryonic cells. A 5' flank sequence of the Nanog gene has been reported to be regulated differentially, and two regulatory elements within the Nanog promoter, namely Oct-4 and Sox-2 binding sites, have been identified to regulate the transcriptional activity of Nanog gene. In this report, we identified the role of two putative Sp1 binding sites located in the Nanog gene 5'-flanking region in regulation of murine Nanog gene transcription. Mutation studies showed that the two sites were essential for the Nanog promoter activity. Gel shift and supershift analysis showed that both sites specifically bind Sp1 and Sp3. Furthermore, overexpression of dominant-negative Sp1 or Sp3 mutants significantly inhibits Nanog promoter activity. These results suggest that the transcription factor Sp1 and Sp3 are important for Murine Nanog gene expression.