Cyclin binding alone causes partial activation of Cdks, but complete activation also requires activating phosphorylation by CAK. In animal cells, CAK phosphorylates the Cdk subunit only after cyclin binding, and so the two steps in Cdk activation are usually ordered as shown here, with cyclin binding occurring first. Budding yeast contains a different version of CAK that can phosphorylate the Cdk even in the absence of cyclin, and so the two activation steps can occur in either order. In all cases, CAK tends to be in constant excess in the cell, so that cyclin binding is the rate-limiting step in Cdk activation.^[1]