Cyclin E belongs to the highly conserved cyclin family, whose members exhibit distinct expression and degradation patterns which contribute to the temporal coordination of each mitotic event. Cyclins function as regulators of CDK kinases. Cyclin E forms a complex with and functions as a regulatory subunit of CDK2, whose activity is required for cell cycle G1/S transition. Cyclin E accumulates at the G1-S phase boundary and is degraded as cells progress through S phase. Cyclin E overexpression has been observed in many tumors, which results in chromosome instability, and thus may contribute to tumorigenesis.