Cyclins are a group of proteins that play a crucial role in regulating the cell cycle, which is the process that cells go through to grow and divide. They help control the timing of cell division by activating specific enzymes called cyclin-dependent kinases (CDKs). When cyclins bind to CDKs, they trigger various events that lead to cell progression through different phases of the cell cycle. The levels of cyclins fluctuate throughout the cell cycle, rising and falling at specific times. 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 M phase (mitosis). This precise regulation ensures that cells divide correctly and maintain proper function.