p27Kip1 is an inhibitor of multiple cyclin-dependent kinases (cdk), and can arrest the cell-cycle progression by inhibiting the phosphorylation of the retinoblastoma gene family products. Tumor formation in p27Kip1 knockout mice clearly shows that p27Kip1 plays an important role in inhibiting tumor formation and progression. To investigate the mechanism of transcriptional p27Kip1 gene expression, we isolated the genomic DNA fragment of the 5' flanking region of the human p27Kip1 gene and characterized its promoter region. The human p27Kip1 promoter is TATA-less, and the sequence is highly homologous to the murine p27Kip1 promoter sequence. In the promoter assay, deletion from -774 to -435 relative to the initiating codon resulted in a 15-20-fold reduction of the p27Kip1 promoter activity, suggesting that the elements for basal promoter activity exist in this highly conserved 340 bp region, where putative CTF and ATF sites are conserved.