CDK2, also known as cyclin-dependent kinase 2, or cell division protein kinase 2, is a member of the cyclin-dependent kinase family of Ser/Thr protein kinases. This protein associates with and is regulated by the regulatory subunits of the complex including cyclin E or A. Cyclin E binds G1 phase CDK2, which is required for the transition from G1 to S phase while binding with Cyclin A is required to progress through the S phase. Abnormal expression of cyclin A2 is associated with chromosomal instability and tumor proliferation, while inhibition leads to decreased tumor growth. Therefore, CDK2 and its cyclin binding partners represent possible therapeutic targets for new cancer therapeutics.