Essential for the control of the cell cycle at the G1/S (start) transition (PubMed:12606285). In association with cdk-2, regulates proliferation, quiescent state and cell fate during the development of several cell lineages (PubMed:10952902, PubMed:17476329, PubMed:21558371, PubMed:21455289). In the embryo, initiates the establishment of cell polarity through the recruitment of the centrosomal proteins spd-2 and spd-5 during prophase (PubMed:17115027). During the development of the vulva, controls the onset of vulval cell terminal differentiation by controlling the duration of G1 phase (PubMed:10952902, PubMed:20005870). During hypoderm development at early larval stages, controls syncytial fate of seam cell daughter cells (PubMed:17476329). Involved in the progression of cell division in the intestinal lineage in larvae, and in particular in endoreplication, a specific growth pathway in the intestinal epithelium, required for feeding and gut development in growing larvae (PubMed:17466069). By controlling the activity of translational repressor gld-1, regulates the pool of germline stem cells and the size of the mitotic zone by preventing entry into meiosis (PubMed:21455289). In addition, repression of expression by gld-1 prevents mitosis re-entry in meiotic germline cells (PubMed:19758560). Interacts with a member of the CDK2/CDK protein kinases to form a serine/threonine kinase holoenzyme complex. The cyclin subunit imparts substrate specificity to the complex (By similarity). Co-localizes with cdk-2 in the sperm centrioles before the first embryonic mitosis and then to the male and female nuclei upon entry into mitosis. Expressed dynamically in proliferating cells throughout development. Detectable in larval blast cells undergoing active proliferation that give rise to all tissue types, including germline, intestine, hypodermis, neurons, and muscle. Expressed both maternally and zygotically. Ubiquitous embryonic pattern of expression declines during embryogenesis and disappears from most cells in comma-stage embryos coincident with the completion of the majority of embryonic cell divisions. Expression levels drop and become restricted and dynamic during postembryonic development (PubMed:12606285). During the development of distal tip cells, expressed asymmetrically between the daughters of the Z1.a and Z4.p cells; asymmetric expression is regulated by wrm-1, a component of the Wnt/MAPK pathway (PubMed:17476329). In the gonads, expression is restricted to the proliferating distal germline cells (PubMed:12606285, PubMed:21558371). In germline cells entering meiosis, expression is repressed by gld-1 (PubMed:21455289, PubMed:19758560). Loss of cell division in vulval lineages as a result of lengthened intervals between cell divisons (PubMed:20005870). RNAi-mediated knockdown results in arrest prior to the 100-cell embryonic stage (PubMed:10952902). Depending on the knockdown, some animals reach adult age (PubMed:17476329). In the 1-cell embryo, persistent ruffling throughout the embryo cortex and mislocalization of par-2, which remains cytoplasmic, and par-6, which is distributed throughout the cortex. In 50 percent of embryos, the first division is symmetric. Adult mutants are sterile due to a lack of sperm and egg production (PubMed:17115027). In mutants and knockdown animals, production of 2 additional distal tip cells (DTC) is often associated with the production of an extra gonad (PubMed:17476329). In addition, gonads of knockdown animals have an abnormal mitotic zone characterized by an enlargement of the distal germ cell nuclei, a reduction in the number of mitotic germ cells, a reduction in the mitotic region length and an abnormal expression of gld-1 (PubMed:21455289). Belongs to the cyclin family. Cyclin E subfamily.