Although there is paucity of circadian genomics study of head and neck pathology, nine circadian clock genes have been investigated in patients with oral squamous cell carcinoma using quantitative real-time polymerase chain reaction and immunohistochemistry, of which PER1 and CLOCK were found differentially expressed in the peripheral blood of those patients and thus could serve as potentially reliable prognostic markers (Hsu et al., 2014). Here, PER1 is linked to oral cavity squamous cell carcinoma.