Furthermore, Pacelli and colleagues quantified the expression of various clock genes (CLOCK, CRY1, CRY2, NR1D1, PER1, PER2, PER3, BMAL1) in cultured fibroblasts from two PD patients with PRKN (Parkin) mutations and found an absence of circadian rhythmicity (period close to 24 h) particularly in CLOCK, PER1 and PER2 in fibroblasts from PD patients [22]. Here, PER2 is linked to Parkinson disease.