The tau mutation in Syrian hamsters shortens circadian rhythms. Researchers identified the tau locus as casein kinase I epsilon (CKIε), a homolog of the Drosophila circadian gene double-time. Mutant CKIε has reduced activity and autophosphorylation, and fails to phosphorylate PERIOD proteins. These findings suggest that tau is a mutant allele of hamster CKIε, which plays a role in circadian regulation. The study used genetic mapping and positional syntenic cloning to identify the tau locus. The tau mutation affects the circadian clock by altering the timing of negative feedback, leading to a shortened period. The study also shows that CKIε is a critical component of the mammalian circadian clock, and its mutation impacts the regulation of PERIOD proteins. The results highlight the importance of CKIε in maintaining normal circadian rhythms and suggest that it could be a target for drugs affecting circadian rhythms, sleep, and jet lag.The tau mutation in Syrian hamsters shortens circadian rhythms. Researchers identified the tau locus as casein kinase I epsilon (CKIε), a homolog of the Drosophila circadian gene double-time. Mutant CKIε has reduced activity and autophosphorylation, and fails to phosphorylate PERIOD proteins. These findings suggest that tau is a mutant allele of hamster CKIε, which plays a role in circadian regulation. The study used genetic mapping and positional syntenic cloning to identify the tau locus. The tau mutation affects the circadian clock by altering the timing of negative feedback, leading to a shortened period. The study also shows that CKIε is a critical component of the mammalian circadian clock, and its mutation impacts the regulation of PERIOD proteins. The results highlight the importance of CKIε in maintaining normal circadian rhythms and suggest that it could be a target for drugs affecting circadian rhythms, sleep, and jet lag.