Objective      To determine the circadian rhythm variations of the expression of clock gene PER1 in oral squamous cell carcinoma (OSCC) and their relations with tumor growth in vivo.       Methods      Sixty nude mice were raised under 12 h light/12 h dark cycles for 3 weeks. Human OSCC cell line BcaCD885 was inoculated in the cheek of nude mice to establish a nude mice model of OSCC. In 3 weeks after the implantation

15 mice we

re sacrificed at 4 time points

including 4 h after light onset (HALO)

10 HALO

16 HALO and 22 HALO

respectively

during a period of 24 h. Tumor tissues were excised and weighed. HE stained sections were prepared and mitotic index (MI) was calculated. The protein and mRNA expression of PER1 in the tumor cells at the 4 time points were determined by immunohistochemical assay (SP method)

Western blot analysis and real-time RT-PCR. The differences of its expression in the 4 time points were assessed by ANOVA

and a cosine analysis method was applied to determine whether the expression of PER1 at protein and mRNA levels obeyed a circadian rhythm.       Results      There were significant differences in PER1 expression at protein and mRNA levels in the OSCC cells

tumor MI

and tumor weight among the 4 time points （

P

0.01）

and these changes were in a circadian rhythm（

P

0.05）. Tumor MI and tumor weight were negatively correlated with the expression level of PER1. The peak of PER1 mRNA expression and the valley values of tumor MI and weight appeared at the middle of active phase. However

the valley of PER1 mRNA expression and the peaks of tumor MI and weight occurred at the middle of rest phase.       Conclusion      PER1 expression

tumor MI

and xenografted tumor weight of OSCC cells show circadian rhythm during a period of 24 h. PER1 is an anti-tumor gene in OSCC.