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 works by activating specific enzymes called cyclin-dependent kinases (CDKs), which are essential for cell cycle progression. The levels of cyclin D in a cell can be influenced by various factors, including growth factors and cellular signals. Abnormal levels of cyclin D are often associated with certain types of cancer, as they can lead to uncontrolled cell division. Understanding cyclin D's function is important for developing targeted therapies in cancer treatment.