The CDKN1B gene provides instructions for making a protein called p27. This protein is found in cells and tissues throughout the body. Within cells, p27 is located primarily in the nucleus, where it plays a critical role in controlling cell growth and division. It helps regulate the cell cycle, which is the cell's way of replicating itself in an organized, step-by-step fashion. Specifically, p27 normally blocks cells from entering the phase of the cell cycle when DNA is copied (replicated) in preparation for cell division. By blocking cell cycle progression, p27 prevents cells from dividing too quickly or at the wrong time. Based on this function, p27 is described as a tumor suppressor protein. Studies suggest that p27 is also involved in controlling cell differentiation, which is the process by which cells mature to carry out specific functions.~Because p27 plays such a key role in controlling cell division, its activity is tightly regulated. Regulation can occur through modification of the p27 protein's structure, its interaction with other proteins, or its localization within the cell. For example, when p27 is held (sequestered) in the fluid that surrounds the nucleus (the cytoplasm) instead of being transported into the nucleus, the protein is unavailable to block cell cycle progression. Researchers believe that p27 may have other functions in the cytoplasm, but these functions have not been well described.