IntroductionA genetic alteration frequently observed in cancer tissue is the increased expression of cyclin E (1). For example, the majority of liver cancers express this protein at levels higher than in the surrounding normal tissue (2). Intriguingly, cyclin E is also able to induce DNA double-strand breaks, which may lead to genetic instability and aneuploid cells (3). Consistent with the importance of cyclin E in regulating cell proliferation and genetic stability, its expression levels are tightly controlled. In addition to transcriptional mechanisms, cyclin E levels are also regulated by posttranslational modifications that trigger the proteolytic degradation of cyclin E by cullin 1-or cullin 3-dependent (Cul1-or Cul3-dependent) ubiquitin ligases. Degradation of cyclin E by the Cul1-dependent ubiquitin ligase requires its interaction with the F-box protein Fbw7, which upon phosphorylation of cyclin E at T380 by GSK3 and T384 by cdk2 tightly binds to the protein and mediates its polyubiquitination and subsequent proteasomal turnover (4, 5). Mutations in Fbw7 have been shown to induce genetic instability, and certain types of human cancers such as T cell leukemias and cholangiocellular carcinomas frequently show alterations in this gene (6).Loss of Fbw7 also affects stem cell proliferation by depleting hematopoietic stem cells due to active cell cycling and an increase in the rate of apoptotic cell death (7-9). Conversely, expression of a stabilized version of cyclin E that can no longer be degraded by the Fbw7-dependent E3 ubiquitin ligase results in an expansion of the