Circadian rhythms rely on the interaction of highly conserved 
transcription-translation loops. Casein kinase I epsilon (CK1epsilon) 
post-transcriptionally regulates circadian rhythms by phosphorylating clock 
genes, and the tau mutation, an arginine to cysteine substitution at residue 
128, results in a short circadian period, abnormal entrainment to light cycles, 
and potentiated resetting responses to light. Each of these effects could be 
attributed to changes in the regulation of the core molecular circadian loops. 
We now demonstrate that the mutation results in a heightened sensitivity to 
light, suggesting that CK1epsilon also regulates the photic entrainment pathway.