Cells use RNA as a versatile tool to regulate the activity of their genes. Small snippets of RNA can fine-tune how much protein is produced from various genes; some small RNAs can shut genes off altogether. An enzyme called Dicer chops RNA into smaller pieces: plants use it to chew up the RNA of invading viruses; worms use it to shut genes off during development; and humans use it to produce gene-regulating microRNAs. Dicer is also of interest because mutations in the gene for the enzyme appear to contribute to some human cancers, although it hasn't been clear exactly why.