In association with cdk-4, regulates the progression through the G1 phase of the cell cycle during postembryonic development (PubMed:10518501, PubMed:11684669, PubMed:25562820). Regulates proliferation of the coelomocyte lineage and intestinal cells during late embryogenesis (PubMed:15708572, PubMed:11684669). In complex with cdk-4, involved in sex determination during gonadogenesis by regulating the asymmetric division of the somatic gonadal precursor cell (SGP) (PubMed:16198291). Interacts with cdk-4; the interaction is likely involved in regulating cdk-4 activity (PubMed:10518501). Expression initiates during mid-embryogenesis primarily in post-proliferative hypodermal cells and neurons in the head, ventral cord and tail, then declines until hatching where it is mainly seen in seam cells. Expressed throughout larval development in several blast cell linages. In the P lineage, expression is restricted to proliferating cells, whereas it persists in somatic gonads and seam cells. In somatic gonads, expression is restricted to the spermathecal cells and their precursors. Not expressed in the intestine. Arrested cell division in the G1 phase during larval development (PubMed:25562820). RNAi-mediated knockdown results in an arrest at L2 stage which is associated with uncoordinated movements, a slightly shorter and stouter body morphology, growth arrest and premature death (PubMed:10518501). In addition, during larval development, cell division is impaired in P blast cells and mesoblast M cells due to an arrest prior to S phase (PubMed:10518501). Lack of cell division in somatic gonad precursors Z1 and Z4 which is associated with a failure to maintain germline proliferation resulting in an abnormal gonad development (PubMed:10518501). At the late embryonic stage, intestinal cells fail to divide without affecting endoreduplication which results in 4n DNA cellular content (PubMed:11684669). Mutants show slightly stronger defects including the development of only two enlarged embryonic coelomocytes (PubMed:11684669, PubMed:15708572). Double knockout with lin-35 rescues the cell cycle progression defect in the single cyd-1 mutant (PubMed:11684669, PubMed:25562820). Belongs to the cyclin family. Cyclin D subfamily.