Large families of RNA molecules of 21-22 nucleotides, called microRNAs, have been found in a number of species. In an Advanced Online Publication from Nature Genetics, Eric Lai from the University of California at Berkley, describes a family of microRNAs in Drosophila (18 March 2002, DOI:10.1038/ng865). He found that 11 Drosophila microRNAs are complementary to the K-box motif (cUGUGAUa), Brd box (AGCUUUA) and GY box (uGUCUUCC) motifs present in the 3' untranslated regions (UTRs) of several basic helix-loop-helix genes (bHLH). The microRNA may bind these sequences and mediate negative post-transcriptional regulation. The conservation of some microRNAs in man and worms will prompt work on defining a regulatory role for microRNA-3'UTR RNA duplexes.