PF-4800567 is a selective, ATP competitive casein kinase 1ε inhibitor with IC50 values of 32 and 711 nM for CK1ε and CK1δ respectively. Casein kinase 1 epsilon (CK1epsilon) and casein kinase 1 delta (CK1delta) are closely related Ser-Thr protein kinases that serve as key clock regulators as demonstrated by mammalian mutations in each that dramatically alter the circadian period. PF-4800567 displays 22-fold greater potency towards CK1ε than CK1δ. PF-4800567 completely blocks CK1epsilon-mediated PER3 nuclear localization and PER2 degradation.