PF-4800567 is a potent and selective small molecule inhibitor of casein kinase 1 epsilon (CK1epsilon), exhibiting an IC50 of 32 nM with over 20-fold selectivity against the closely related CK1delta isoform. This tool compound is used in research to investigate circadian rhythms, Wnt/beta-catenin signaling, and neurodegenerative diseases in both cellular and animal models.