Essential for the control of the cell cycle at the late G1 and early S phase. Interacts with the CDK2 (in vivo) and CDK3 (in vitro) protein kinases to form a serine/threonine kinase holoenzyme complex. The cyclin subunit imparts substrate specificity to the complex. According to PubMed:9858585, highest levels of expression in adult testis, thymus and brain. Lower levels in placenta, spleen and colon. Consistently elevated levels in tumor-derived cells compared to non-transformed proliferating cells. According to PubMed:9840927: low levels in thymus, prostate, brain, skeletal muscle, and kidney. Elevated levels in lung. According to PubMed:9840943 highly expressed in testis, placenta, thymus and brain. In a lesser extent in small intestine and colon. Activated by papilloma viral oncoproteins E6 and E7 which bind to and inactivate p53 and Rb, respectively. Phosphorylation by CDK2 triggers its release from CDK2 and degradation via the ubiquitin proteasome pathway. Belongs to the cyclin family. Cyclin E subfamily.