Essential for the control of the cell cycle at the G1/S (start) transition. Interacts with CDK2 protein kinase to form a serine/threonine kinase holoenzyme complex. The cyclin subunit imparts substrate specificity to the complex (By similarity). Phosphorylation by CDK2 triggers its release from CDK2 and degradation via the ubiquitin proteasome pathway. Belongs to the cyclin family. Cyclin E subfamily.