Transcriptional/translational feedback loops between clock genes such as per and tim - first cloned in Drosophila - and their protein products are thought to constitute the core of the circadian clock. A representative of the early metazoan body plan, the nematode (roundworm) Caenorhabditis elegans has had its complete cell lineage traced and it is known that it has many molecular components in common with those of the vertebrates. Recent studies have shown that C. elegans carries a per hmologue,
lin-42, and that its mRNA levels oscillate synchronously with approximately 6 hr molting cycles at the postembyronic developmental stage.