Cyclin-dependent kinase inhibitor function as tumor suppressor proteins by inhibiting the enzyme cyclin-dependent kinase. The enzyme cyclin-dependent kinase controls cell cycle progression, transcription, mRNA processing, and differentiation. Cyclin-dependent kinase inhibitor arrest cell cycle at the G1 phase leading to mitosis. Cyclin-dependent kinase inhibitors are employed in the treatment of cancers by preventing over proliferation of cancer cells. Cyclin-dependent kinase are very active in most cancers which is rational to the use of cyclin-dependent kinase inhibitor as anti-cancer agents.