PER2 and central nervous system cancer: The differential expression of CCGs in glioma grading confirmed that cluster I genes (ARNRL, NPAS2, and TIMELESS) and CRY1, with cluster II genes (CRY2, DBP, NR1D1, NR1D2, PER2, PER3, and RORA) showed significantly opposite expression trends in brain tissues (Figure 3).