PER2 and cancer: Patients diagnosed with a head and neck squamous cell carcinoma showed a significant dysregulation of circadian clock core components BMAL1, CRY1, CRY2, PER1, PER2, PER3 and CASEIN KINASE 1ε (CK1ε), depending on the stage of tumor development as advanced cancer stages correlated with downregulation of BMAL1, CRY2 and PER3 [35].