Cell cycle: will the real Cdk-activating kinase please stand up.

Activation of a cyclin-dependent kinase (Cdk) requires its association with a cyclin subunit and phosphorylation on a conserved residue by the Cdk-activating kinase (CAK). The discovery in budding yeast of a novel CAK that is structurally distinct from the CAKs of other eukaryotes raises questions about the 'true' CAK.