Cyclin E1 is a regulatory subunit of the CDK2 kinase. It is central for the regulation of the G1/S transition and its abundance is tightly regulated throughout the cell cycle via ubiquitination. Cyclin E has been the subject of intense study in relation to tumorigenesis and cancer management and prognosis. Multiple isoforms of Cyclin E are only expressed in tumors but not in normal tissue, suggesting a post transcriptional regulation of Cyclin E. In vitro analyses indicated that these truncated variant isoforms of Cyclin E are able to phosphorylate histone H1. Alterations in the Cyclin E protein have been implicated as indicators of worse prognosis in various cancers.