Circadian rhythms: In the loop at last

The basic molecular mechanisms underlying circadian oscillators follow the same general plan across the phylogenetic spectrum: oscillating feedback loops in which clock gene products negatively regulate their own expression. The circadian clocks of animals involve at least two interacting feedback loops. This Viewpoint compares and contrasts the circadian clocks of mammals and of Drosophila, emphasizing how different players are used to create the same basic script. Both the general script and the specific details of the murine and Drosophila circadian pathways are available at Science's Signal Transduction Knowledge Environment Connections Maps.