Cyclin E1 and cyclin E2 can associate with and activate cdk2 (1). Upon DNA damage, upregulation/ activation of the cdk inhibitors p21Waf1/Cip1 and p27Kip1 prevents cyclin E/cdk2 activation, resulting in G1/S arrest. However, when conditions are favorable for cell cycle progression, cyclin D/cdk4/6 phosphorylates Rb and is thought to reduce the activity of p21Waf1/Cip1 and p27Kip1, allowing subsequent activation of cyclin E/cdk2 (1,2). Cyclin E/cdk2 then further phosphorylates Rb to allow progression into S-phase, where cyclin E/cdk2 is thought to phosphorylate and activate multiple proteins involved in DNA synthesis (2,3). Turnover of cyclin E is largely contolled by phosphorylation resulting in SCFFbw7-mediated ubiquitination and proteasomedependent degradation (4,5). Cyclin E1 is phosphorylated at multiple sites in vivo including Thr62, Ser88, Ser72, Thr380 and Ser384, and is controlled by at least two kinases, Cdk2 and GSK3 (6,7).