Here's why:

* Cyclins: These are regulatory proteins that fluctuate in concentration throughout the cell cycle. Their levels rise and fall in a cyclical pattern.

* Cyclin-dependent kinases (CDKs): These are enzymes that require a cyclin partner to become active. They are responsible for phosphorylating (adding a phosphate group to) other proteins, which triggers various events in the cell cycle.

Together, cyclins and CDKs form complexes that act as key regulators of cell cycle progression. They ensure that each phase of the cell cycle is completed before the next one begins, preventing errors and maintaining the integrity of the genome.

Let me know if you'd like to learn more about specific cyclins and CDKs, their roles in different stages of the cell cycle, or the regulation of their activity!