Altered expression of PER1, PER2 and/or PER3 have been reported in colorectal, pancreatic, gastric, oral, breast, prostate, bladder, renal, and non-small cell lung cancers, as well as in glioma, hepatocellular carcinoma, head and neck squamous cell carcinoma and myeloid leukemia [75,76,77,78,79,80,81,82,83,84,85,86,87]. Here, PER2 is linked to central nervous system cancer.