Essential for the control of the cell cycle at the G1/S (start) transition. In association with cdk-2, regulates proliferation, quiescent state and cell fate during the development of several cell lineages. In the embryo, initiates the establishment of cell polarity through the recruitment of the centrosomal proteins spd-2 and spd-5 during prophase. During the development of the vulva, controls the onset of vulval cell terminal differentiation by controlling the duration of G1 phase. During hypoderm development at early larval stages, controls syncytial fate of seam cell daughter cells. 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. 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. In addition, repression of expression by gld-1 prevents mitosis re-entry in meiotic germline cells. 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. Belongs to the cyclin family. Cyclin E subfamily.