The award of this year's Nobel Prize for Physiology or Medicine to Leland Hartwell, Tim Hunt and Paul Nurse — for their discoveries of 'key regulators of the cell cycle' — is a testimony to the importance of this area. The molecular mechanisms that control progression of the cell cycle are implicated in a variety of processes, from development and tissue homeostasis to tumour formation.

One aspect of cell-cycle regulation is discussed by Allison J. Bardin and Angelika Amon on page 815 of this issue. They describe two signalling cascades — the mitotic-exit network (MEN) and the septation-initiation network (SIN) — that control key events during exit from mitosis and cytokinesis, respectively. The MEN is found in the budding yeast Saccharomyces cerevisiae, whereas the SIN is found in Schizosaccharomyces pombe (fission yeast), yet the two networks contain many common components.

Bardin and Amon ask how, given this apparent conservation between their components, the two pathways have such different outcomes. Whereas the SIN controls the formation of the division septum after mitotic cyclin-dependent kinases (CDKs) have been inactivated, the MEN is involved in regulating the mitotic CDK inactivation itself. But Bardin and Amon argue that there is evidence for a role of the MEN in regulating cytokinesis, as well as hints that the SIN could, under certain circumstances, control the activity of mitotic CDKs.

The idea that similar proteins can have different functions in various organisms is gaining much support. But functional conservation between species is, perhaps, more common, as illustrated by the Highlight on page 785. Also focusing on the cell cycle, this article summarizes the results from several groups who have shown that the cyclin E protein is targeted for proteolytic destruction by an F-box protein found in yeast, Drosophila and humans.