Which of the following is not a cyclin dependent kinase (CDK) inhibitor?

Cyclins form complex with cyclin dependent kinases and regulate the transition of cell cycle from one stage to the other. These CDK complexes in turn are regulated by CDK inhibitor. The inhibitors control the cell cycle by balancing the activity of CDKs. The signals from these inhibitors determine whether a cell progresses through the cell cycle. Changes in the level of these inhibitors may occur in some tumors, or possibly in aging cells. The cyclin dependent kinase inhibitors are - p21 - p27 - p57 - p15 - p16 - p18 - p19.