Circadian transcription: passing the HAT to CLOCK.

In animals, the circadian timekeeping mechanism relies on the coordinated activities of activators and repressors to control rhythmic transcription. In this issue of Cell, Doi et al. (2006) reveal that rhythms in histone acetylation are necessary for rhythmic transcription and that the histone acetyl transferase responsible is CLOCK, a key transcription factor that is essential for circadian oscillator function.