Potent tight-binding inhibitor of several G1 cyclin/CDK complexes (cyclin E-CDK2, cyclin D2-CDK4, and cyclin A-CDK2) and, to lesser extent, of the mitotic cyclin B-CDC2. Negative regulator of cell proliferation. May play a role in maintenance of the non-proliferative state throughout life. Interacts with PCNA. Expressed in the heart, brain, lung, skeletal muscle, kidney, pancreas and testis. High levels are seen in the placenta while low levels are seen in the liver. Belongs to the CDI family.