Comparing the gene expression difference between normal cultured cells and serum starvation cells with DD-PCR method，a cDNA fragment, which expresses increase under serum starvation, was obtained. According to this cDNA fragment, the full gene cDNA was obtained primarily by searching and splicing the EST sequences, then was cloned by PCR and sequenced. The size of this gene is 5 429 bp and it codes a peptide containing 791 amino acid residues. It is not homologous with known genes that related to cell cycle. So, this gene may be a new cell cycle-related gene(GenBank: AY050169). It was named as serum inhibition gene (Si-1) according to its expression inhibited under serum existing.