Heterochronic protein which controls the choice of stage specific cell fates (PubMed:9054503, PubMed:6494891, PubMed:2702689, PubMed:2628162, PubMed:1916265, PubMed:7671811, PubMed:8625405, PubMed:8756295, PubMed:8756296, PubMed:9477318, PubMed:9649524, PubMed:10706289, PubMed:12871707, PubMed:15073154, PubMed:16139228). Regulates the timing of the second larval stage events (L2 events) in the hypodermis (PubMed:2702689, PubMed:2628162). May negatively regulate the larval to adult transition via the suppression of the microRNA (miRNA) let-7 during L3 (PubMed:10706289, PubMed:12871707, PubMed:16139228). Through this regulatory role, controls the timing of the sexual maturation of the nervous system (PubMed:31264582). Also has a role in the fox-1-sex-1-mediated determination of sexual fate (PubMed:21471153). Plays a role in governing the developmental timing of male tail tip morphogenesis (PubMed:26811380, PubMed:30956008). Plays a role in controlling the seam cell number during larval stages (PubMed:21471153, PubMed:28602583). Plays a role in vulval development (PubMed:28602583). Component of a complex at least containing lep-2, lin-28 and the long non-coding RNA lep-5, which mediates the degradation of lin-28. Expressed at the first larval stage (L1) in several cell types including hypodermis, muscle, neurons and seam cells (PubMed:9054503, PubMed:11884032, PubMed:16122423, PubMed:16139228, PubMed:26811380, PubMed:31264582, PubMed:30956008, PubMed:21471153). Also expressed in the tail tip and pharynx at the L1 stage (PubMed:26811380). Down-regulated at L2, and absent from L3 and L4 (PubMed:9054503, PubMed:11884032, PubMed:16122423, PubMed:16139228, PubMed:26811380, PubMed:31264582, PubMed:30956008, PubMed:21471153). Negatively regulated by the miRNA lin-4 which causes degradation of the mRNA encoding this protein. This requires a lin-4 complementary element (LCE) in the 3'-UTR of the mRNA encoding this protein. Also negatively regulated independent of lin-4 and this is counteracted by the action of lin-14. Positively regulated by TGF-beta signaling. Cleavage by caspase ced-3 during larval development probably induces lin-28 degradation. RNAi-mediated knockdown in a ced-3 and ain-1 double mutant background reduces the percentage of animals with developmental defects including impaired egg-laying and production of ectopic seam cells. RNAi-mediated knockdown results in the precocious onset of tail tip retraction at the L3 larval stage resulting in over-retracted and shortened adult male tails (also known as the Ore phenotype) (PubMed:26811380, PubMed:30956008). RNAi-mediated knockdown suppresses the male tail tip morphogenesis defects of mutants for the long non-coding RNA lep-5 (PubMed:30956008). Belongs to the lin-28 family.