A small fortune: small RNAs involved in the regulation of developmental timing

The world of small RNAs just got bigger. In Caenorhabditis elegans, two small temporal RNAs produced by the ribonuclease Dicer have previously been shown to be involved in regulating developmental timing. These RNAs — lin-4 and let-7, 22 and 21 nucleotides in length, respectively — act as antisense repressors of messenger RNA translation and, until recently, they were the only known RNAs of this class. But three papers published in Science now show that lin-4 and let-7 probably belong to a large family of single-stranded RNAs, 20 24 nucleotides in length, called microRNAs (miRNAs). These results indicate that post-transcriptional regulation by small RNAs is more common than previously believed.