Abstract
Period (PER) proteins are essential parts of the molecular clocks that control circadian rhythms in flies and mammals. Phosphorylation regulates PER's stability and subcellular localization; however, the physiologically relevant sites have been difficult to identify in spite of knowing the relevant kinase. In this issue of Genes & Development, Chiu and colleagues (1758-1772) identify a key phosphorylation site on PER that recruits the F-box protein Slimb to trigger PER degradation and set clock speed.
