The main circadian clock in mammals is located in the brain, but the cells of many peripheral tissues also contain clocks. Yagita et al. report that the molecular components that make up these peripheral clocks, at least those in fibroblasts, are the same as those that form the brain's master clock in the mouse. Transcriptional and translation feedback loops consisting of Per2, Bmal1, Clock, and Cry1, and their respective proteins, interact to produce a circa 24-hour rhythm in cultured fibroblasts. The oscillations and period length are dependent on the presence of Cry1, as in the brain. Thus, the causes of the distinctive features of peripheral oscillators--a tendency to exhibit dampening and insensitivity to light--must be sought elsewhere.