The Period2 gene, an indispensable component of the circadian clock, not only 
modulates circadian oscillations, but also regulates organic function. We 
examined whether overexpression of the mouse Period2 gene (mPer2) in tumor cells 
influences cell growth and induces apoptosis. Overexpression of PERIOD2 in the 
mouse Lewis lung carcinoma cell line (LLC) and mammary carcinoma cell line 
(EMT6) results in reduced cellular proliferation and rapid apoptosis, but not in 
NIH 3T3 cells. Overexpressed mPER2 also altered the expression of 
apoptosis-related genes. The mRNA and protein levels of c-Myc, Bcl-X(L) and 
Bcl-2 were downregulated, whereas the expression of p53 and bax was upregulated 
in mPER2-overexpressing LLC cells compared with control cells transferred with 
empty plasmid. Our results suggest that the circadian gene mPeriod2 may play an 
important role in tumor suppression by inducing apoptotic cell death, which is 
attributable to enhanced pro-apoptotis signaling and attenuated anti-apoptosis 
processes.