Hence, it has been hypothesized that circadian clock genes can regulate or suppress other circadian genes by certain mechanisms, which may also corroborate the finding on simultaneous lower expression levels of circadian gene such as PER1, PER2, PER3, CRY1, CRY2 and BMAL1 in HNSCC (Table 2) [5]. This evidence concerns the gene PER2 and head and neck squamous cell carcinoma.