Lin-28: An early embryonic sentinel that blocks Let-7 biogenesis

microRNAs are small regulatory RNAs that are processed from larger, genomically encoded transcripts. While the biochemical mechanism underlying microRNA processing is well understood, it was recently discovered that processing of one developmentally crucial group of microRNAs, the Let-7 family, is blocked by the protein Lin-28 in embryonic cells. This novel regulation of microRNA biogenesis may be very important for the maintenance of embryonic stem cell pluripotency as well as for the reprogramming of somatic cells to induce pluripotent stem cells. The studies leading to the discovery of the Let-7 block by Lin-28 and questions regarding the biochemical mechanism behind Lin-28-mediated microRNA silencing are discussed.