By fusing a hammerhead ribozyme to a small nucleolar RNA, researchers have created an artificial ribozyme, dubbed a "snorbozyme," which localizes to the nucleolus of a yeast cell and cleaves a target RNA with nearly perfect efficiency. The new work (PNAS 96, 6609–6614, 1999) may provide an approach to gene control for both basic research and clinical use. Hammerhead ribozymes can be engineered to cleave almost any RNA molecule, but the relatively low efficiency of artificial ribozymes in vivo has limited their use. Reasoning that co-localization of a ribozyme and its RNA substrate in the same cellular compartment would result in more efficient cleavage, the scientists used small nucleolar RNAs (snoRNAs) as both catalysts and targets. The snorbozyme is metabolically stable, and is directed to the nucleolus with high specificity. Kinetic analysis showed that the snorbozyme cleaves its target snoRNA with an efficiency approaching 100%, functionally knocking out the snoRNA gene. Maurille Fournier, a senior author on the paper, says that as many as 200 different snoRNAs could be targeted with this approach, and it may also be possible to modify the technique to cleave specific mRNA molecules in the nucleoplasm. "As a research tool, there are just lots of wonderful opportunities for selectively knocking out genes and gene products in a natural cellular environment," says Fournier.