PER2 and central nervous system cancer: In glioma tissue, the levels of Per1 and Per2 mRNA were significantly higher in the irradiated group than in the control group when Per1 and Per2 expression was elevated, i.e. at ZT4 for Per1 (t = –4.464, p < 0.001) and ZT0 for Per2 (t = –2.407, p < 0.05).