PER2 and central nervous system cancer: A range of studies have already confirmed the potential link between brain tumors and CCGs, including Bmal1, Clock, Period 1, Period 2, Period 3, Cry1, Cry2, retinoid-related orphan receptor-α (ROR-α). Growing evidence regarding gliomas including glioblastoma multiforme (GBM) indicates the significance of modulation of CCG in cancer biology.