Cyclin E is a regulatory subunit of Cdk2 and controls G1 / S transition during the mammalian cell cycle. 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.