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 (PubMed:15660127). Found in a complex with CDK2, CABLES1 and CCNA1 (By similarity). Part of a complex consisting of UHRF2, CDK2 and CCNE1 (PubMed:15178429). Interacts directly with UHRF2; the interaction ubiquitinates CCNE1 and appears to occur independently of CCNE1 phosphorylation (PubMed:21952639). Interacts with INCA1 (PubMed:21540187). Highly expressed in testis and placenta. Low levels in bronchial epithelial cells. Phosphorylation of both Thr-395 by GSK3 and Ser-399 by CDK2 creates a high affinity degron recognized by FBXW7, and accelerates degradation via the ubiquitin proteasome pathway. Phosphorylation at Thr-77 creates a low affinity degron also recognized by FBXW7. Ubiquitinated by UHRF2; appears to occur independently of phosphorylation. Lacks 49 residues within the cyclin box and cannot complex with CDK2. Belongs to the cyclin family. Cyclin E subfamily.