The activity of cyclin dependent kinase 2 is regulated during cell cycle. It is first activated by cyclin binding. Cyclin has a cyclin box, followed by a region with another cyclin box-like structure. Then it is further activated by the phosphorylation at the activation loop by CDK activation kinase. On the other hand, CDK5 is not regulated by cyclins, nor it is regulated during cell cycle. Instead, it is robustly activated by p25, which has only one cyclin box-like structure.
