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.