Tiny RNA molecules called microRNAs are important in development, and are thought to function by causing the degradation of matching messenger RNAs. That may not be their only mode of action, however. RNA molecules come in various sizes, ranging from the very long to the very short. The smaller RNAs fall into two major classes: microRNAs (miRNAs), which guide development in an organism by regulating target genes'; and small interfering RNAs (siRNAs), which target viruses, inserted genes and mobile genetic elements —a significant function being defence of the genome. One way in which siRNAs work is by guiding the modification (by methyla-tion) of DNA strands, as well as the modification of the histone proteins around which DNA is wrapped, thus silencing gene expression. Writing in Developmental Cell, Bao et al suggest that miRNAs might also — contrary to expectation — contribute to DNA methylation.
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