Cyclin: The kinases that drive the cell cycle are actually present at a constant concentration in the growing cell, but much of the time they are in an inactive form. To be active, such a kinase must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell. Because of the requirement, these kinases are called cyclin-dependent kinases, or Cdks. The activity of a Cdk rises and falls with changes in the concentration of its cyclin partner. (Campbell & Reece AP Biology, Seventh Edition)