In peripheral blood samples of a large cohort of PD patients (N = 153) and healthy controls (N = 156), a significant reduction of BMAL1 (also known as ARNTL), CLOCK, and PER1 expression was observed [24], and decreased expression of BMAL1 validated in whole-blood samples (N = 30 patients, and N = 15 controls) collected over a time-course of 24 hours with a 3 hour-sampling frequency [16]. Here, BMAL1 is linked to Parkinson disease.