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.
Draetta, G. F. (2004). Cell cycle: Will the real Cdk-activating kinase please stand up. Current Biology, 7(1), R50–R52. https://doi.org/10.1016/s0960-9822(06)00020-0