The article "Positional Syntenic Cloning and Functional Characterization of the Mammalian Circadian Mutation tau" describes the identification and characterization of the *tau* mutation in Syrian hamsters, which causes a shortened circadian period. Using genetically directed representational difference analysis (GDRDA), the researchers identified the *tau* locus as being located on the hamster chromosome 22, corresponding to the mouse gene *Celsr1* and human chromosome 8. Further analysis revealed that the *tau* mutation is an allele of the hamster casein kinase I epsilon (CKIε) gene, which is homologous to the *Drosophila* circadian gene *dbt*. The mutation results in a single base-pair substitution, leading to an arginine-to-cysteine amino acid substitution at residue 178, which affects the enzyme's maximal velocity and autophosphorylation state. The mutant CKIε enzyme is deficient in its ability to phosphorylate PERIOD proteins, which are crucial for the circadian feedback loop. Functional studies show that the *tau* mutation leads to a shortened circadian period by altering the timing of the negative feedback loop, resulting in earlier repression of the CLOCK-BMAL complex. The study highlights the role of CKIε in the circadian clock system and its potential as a target for pharmacological interventions related to circadian rhythms, sleep, and jet lag. The findings provide a deeper understanding of the molecular mechanisms underlying circadian regulation in mammals.The article "Positional Syntenic Cloning and Functional Characterization of the Mammalian Circadian Mutation tau" describes the identification and characterization of the *tau* mutation in Syrian hamsters, which causes a shortened circadian period. Using genetically directed representational difference analysis (GDRDA), the researchers identified the *tau* locus as being located on the hamster chromosome 22, corresponding to the mouse gene *Celsr1* and human chromosome 8. Further analysis revealed that the *tau* mutation is an allele of the hamster casein kinase I epsilon (CKIε) gene, which is homologous to the *Drosophila* circadian gene *dbt*. The mutation results in a single base-pair substitution, leading to an arginine-to-cysteine amino acid substitution at residue 178, which affects the enzyme's maximal velocity and autophosphorylation state. The mutant CKIε enzyme is deficient in its ability to phosphorylate PERIOD proteins, which are crucial for the circadian feedback loop. Functional studies show that the *tau* mutation leads to a shortened circadian period by altering the timing of the negative feedback loop, resulting in earlier repression of the CLOCK-BMAL complex. The study highlights the role of CKIε in the circadian clock system and its potential as a target for pharmacological interventions related to circadian rhythms, sleep, and jet lag. The findings provide a deeper understanding of the molecular mechanisms underlying circadian regulation in mammals.