(24), examined the relation between fatigue and the circadian rhythm genes ARNTL2, CLOCK, PER1, and PER2 in glioma patients; they found that SNPs in ARNTL2 (rs922270) (OR: 1.869, p =0.033) and PER2 (rs934945) (OR: 0.614, p <0.100) were associated with fatigue. Here, PER2 is linked to central nervous system cancer.