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The tau mutation in hamsters was recognized more than 12 years ago when a shipment of Syrian hamsters were found to have a short-period circadian rhythm of 20 hours. As Young explains in a Perspective, although the intervening 12 years has revealed that the product of the tau gene is a crucial player in the intricate workings of the mammalian clock, its identity has remained a mystery. Now new findings ( Lowrey et al.) definitively show that tau encodes a casein kinase Ie and is a homolog of the clock gene double-time in the fruit fly. In wild-type hamsters, the casein kinase Ie phosphorylates the crucial circadian clock protein, PER, which is retained in the cytoplasm and degraded. In tau hamsters, phosphorylation of PER is defective and so PER may be able to move to the nucleus and repress transcription of per genes prematurely, speeding up the circadian clock.