The Caenorhabditis elegans lin-4 and let-7 genes encode small RNAs that bind to complementary sequences in the 3' untranslated region of various developmental genes. Both genes control developmental timing, with let-7 driving a transition from late larval to adult cell fates. In the 2 November Nature, Pasquinelli et al. report that homologs of let-7 (but not lin-4) are found in a wide range of bilaterian animals, including flies, abalone, sea urchins, sea squirts, zebrafish, frog and human (Nature 2000, 408:86-89). The timing of let-7's appearance suggests it may share a developmental function in these other organisms. In flies the let-7 RNA appears at the late third instar, just before metamorphosis, and in the zebrafish it appears between 24 and 48 hours after fertilization. The let-7 sequence is not evident in unicellular organisms or plants.

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