Two autosomal semidominant mutations that affect the period of the circadian clock have been discovered in rodents. In the golden hamster, the tau mutation shortens the period from 24 h (wild-type) to 22 h (heterozygote) and 20 h (homozygote). In the mouse the Clock mutation lengthens the period by more than 1 h (heterozygote) and initially by about 4 h in homozygotes, which then become arrhythmic within 3 weeks. These mutations, and the others that will surely be identified by further screening, will provide powerful tools for the dissection of the mammalian clock at both the molecular and the system level.
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