Coordinated transcription of key pathways in the mouse by the circadian clock

Before this study, the body clock was understood mainly as a small set of genes that oscillate in the brain. What nobody knew was how far the schedule extended into the rest of the genome, or how much of the body it actually governed.

The approach was to measure the activity of thousands of genes at repeated points around the clock, in both the brain's master pacemaker and the liver. Reading the whole genome at several times of day, rather than at one convenient moment, was the methodological step that made everything else visible.

Roughly a tenth of all genes turned out to rise and fall on a daily cycle. That alone reframed the clock from a niche mechanism into something that organises a large part of physiology. The second finding was more surprising. The cycling genes in the liver were mostly not the same genes as those cycling in the brain. Each tissue keeps its own schedule, tuned to its own job, and the pathways involved covered much of core metabolism.

Two consequences followed. Circadian biology became a whole body subject rather than a brain subject, since a liver cell or a gut cell keeps time as surely as a neuron does. And because so much of metabolism proved to be scheduled, the hour at which a medicine is taken became a legitimate scientific question. This paper is the foundation of the large tissue by tissue mapping work that came after it.