The molecule 8-Oxo-7, 8-dihydroguanine (8-oxoGua), an oxidized form of guanine, can pair with adenine or cytosine during nucleic acid synthesis.RNA containing 8-oxoGua causes translational errors leading to abnormal proteins.Human NUDT5, a MutT-related protein, catalyzes the hydrolysis of 8-oxoGDP to 8-oxoGMP, thereby preventing misincorporation of 8-oxoGua into RNA.To investigate the biological roles of NUDT5 in mammalian cells, we established cell lines with decreased levels of NUDT5 expression.In NUDT5 knockdown cells, the RNA oxidation level was not significantly higher than that of normal cells, howerver, the cell cycle G1 phase was significantly delayed, and cell numbers in both the S and G2/M phases were reduced, indicating that cell proliferation was hampered by NUDT5 knockdown.Key proteins for preventing the G1-S transition, such as p53, p16 and Rb were increased, while the phosphorylation of Rb was decreased.These results suggested that the NUDT5 protein may play significant roles in regulating the G1-S transition in mammalian cells.