Cyclin dependent kinases (CDKs) are proline-directed serine/threonine kinases that play a central role in regulating cell cycle progression. Since the activity of CDKs depends on the binding of regulatory subunit named cyclins, the analysis of the CDK functions that have been studied so far has usually gone in parallel to the study of cyclins. However, the proteins referred to as RINGO or Speedy, which have no amino acid sequence homology to cyclins, have been found to function as atypical CDK activators.