Cyclins are a family of proteins that play a crucial role in regulating the cell cycle, which is the series of events that lead to cell division and replication. They work by activating cyclin-dependent kinases (CDKs), enzymes that help control the progression of the cell cycle at various checkpoints. The levels of cyclins fluctuate throughout the cell cycle, rising and falling at specific stages. For example, cyclin D is important for the transition from the G1 phase to the S phase, while cyclin B is essential for the transition from the G2 phase to mitosis. This precise regulation ensures that cells divide correctly and maintain proper function.