Mammalian peripheral tissues have circadian clocks that keep time by generating daily rhythms of transcription. In an Advanced Online Publication in Nature, Kai-Florian Storch and colleagues at Harvard Medical School, report a comparative analysis of the circadian variation in gene expression in mouse liver and heart (Nature 2002, DOI 10.1038/nature744).

Storch et al. used oligonucleotide microarrays to follow changes in the expression of over 12,000 genes over a 24 hour period. They applied a filtering procedure to eliminate 'noise', including use of a set of 'guide genes' that are known to exhibit circadian regulation; this allowed them to zoom in on a subset of genes (575 in liver and 462 in heart) that appear to exhibit circadian oscillations. They estimate that 8-10% of expressed genes show circadian regulation.

When they compared the sets they found very little overlap, with only 52 genes being common to both...