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. Part of a complex consisting of UHRF2, CDK2 and CCNE1. Interacts directly with UHRF2; the interaction ubiquitinates CCNE1 and appears to occur independently of CCNE1 phosphorylation (By similarity). Found in a complex with CDK2, CABLES1 and CCNA1 (PubMed:11585773). Interacts with INCA1 (By similarity). Found in adult spleen, and to a lesser extent in adult testis and brain. Phosphorylation of both Thr-393 by GSK3 and Ser-397 by CDK2 creates a high affinity degron recognized by FBXW7, and accelerates degradation via the ubiquitin proteasome pathway. Phosphorylation at Thr-74 creates a low affinity degron also recognized by FBXW7 (By similarity). Ubiquitinated by UHRF2; appears to occur independently of phosphorylation. Belongs to the cyclin family. Cyclin E subfamily.