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 (By similarity). Highest levels in adult testis, thymus and brain. Lower levels in placenta, spleen and colon. Phosphorylation by CDK2 triggers its release from CDK2 and degradation via the ubiquitin proteasome pathway. Belongs to the cyclin family. Cyclin E subfamily.