Cyclin D is a protein that plays a crucial role in regulating the cell cycle, particularly during the transition from the G1 phase to the S phase. It helps control the progression of cells from resting to dividing, ensuring that cells replicate their DNA at the right time. Cyclin D levels fluctuate throughout the cell cycle, rising in response to growth signals. When Cyclin D binds to specific enzymes called cyclin-dependent kinases (CDKs), it activates them, leading to the phosphorylation of target proteins. This process is essential for cell division and growth, making Cyclin D important in normal cellular function and development. Abnormal levels of Cyclin D can be associated with various cancers.