Essential for the control of the cell cycle at the G1/S (start) transition. Targeted by archipelago for degradation by the SFC ubiquitin ligase complex. Interacts with a member of the CDK2/CDK protein kinases to form a serine/threonine kinase holoenzyme complex (PubMed:11565033). The cyclin subunit imparts substrate specificity to the complex (PubMed:11565033). Interacts (via C-terminus) with Z600 (via C-terminus) (PubMed:17431409). Isoform II is ubiquitous in early embryos and, prior to mitosis 14, is rapidly degraded in all cells except the pole (germ) cells. Expressed during G1 phase in proliferating peripheral nervous system cells. Constitutive expression in embryonic cycles lacking a G1 phase. Isoform II is expressed maternally and isoform I zygotically in larvae. Belongs to the cyclin family. Cyclin E subfamily.