CLOCK and hepatocellular carcinoma: The present study has found that the mRNA levels of Per1, Per2, Per3 and Cry2 in HCC cancerous tissues were significantly lowered compared with the paired peritumoral tissues, while no significant difference was observed in the expression levels of CLOCK, BMAL1, Cry1 and CK1ɛ. This finding was consistent with the study by Lin et al (19), which revealed that the expression of Per1, Per2, Per3 and Cry2 was lower in HCC tissue samples.