Stopping time: The genetics of fly and mouse circadian clocks

Forward genetic analyses in flies and mice have uncovered conserved transcr iptional feedback loops at the heart of circadian pacemakers. Conserved mec hanisms of posttranslational regulation, most notably phosphorylation, appe ar to be important for timing feedback. Transcript analyses have indicated that circadian clocks are not restricted to neurons but are found in severa l tissues. Comparisons between flies and mice highlight important differenc es in molecular circuitry and circadian organization. Future studies of pac emaker mechanisms and their control of physiology and behavior will likely continue to rely on forward genetics.