In this study, we aim to address such open questions by a detailed characterization of the circadian expression profiles of core-clock genes (e.g., BMAL1, CRY2, PER2, NR1D2) and clock-controlled genes (e.g., IL-6, AKT1) obtained from saliva samples of PD patients and non-PD controls across two consecutive days. The gene discussed is PER2; the disease is Parkinson disease.