Cyclin-dependent kinase 2 also known as cell division protein kinase 2[1]. The protein encoded by this gene is a member of the cyclin-dependent kinase family of Ser/Thr protein kinases. This protein kinase is highly similar to the gene products ofS. cerevisiaecdc28, andS. pombecdc2, also known as Cdk1 in humans. It is a catalytic subunit of the cyclin-dependent kinase complex, whose activity is restricted to the G1-S phase of the cell cycle, and is essential for the G1/S transition. 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. Its activity is also regulated by phosphorylation. Two alternatively spliced variants and multiple transcription initiation sites of this gene have been reported[2]. The role of this protein in G1-S transition has been recently questioned as cells lacking Cdk2 are reported to have no problem during this transition[3].

References:
1. Lawrie, A. M.; Noble, M. E.; Tunnah, P.; Brown, N. R.; Johnson, L. N.; Endicott, J. A. (1997). "Protein kinase inhibition by staurosporine revealed in details of the molecular interaction with CDK2". Nature structural biology 4 (10): 796–801
2. Tsai LH, HarlowE, Meyerson M (September 1991). "Isolation of the human cdk2 gene that encodes the cyclin A- and adenovirus E1A-associated p33 kinase". Nature 353 (6340): 174–7.
3. Berthet C, Aleem E, Coppola V, Tessarollo L, Kaldis P (October 2003). "Cdk2 knockout mice are viable". Curr. Biol. 13 (20): 1775–85.