PER2 and central nervous system cancer: Niu et al. used an animal model of brain glioma to study the expressions of Per1 and Per2 in normal and glioma tissues, and determined that the expression of Per1 was at its minimum at midnight, while the expression of Per2 is maximal at midnight and minimum at 8:00 a.m. in glioma tissues (Zhanfeng et al., 2015).