The vast majority of mammalian microRNAs originate from a surprisingly small number of microRNA genes, according to an atlas of microRNA expression published this week in Cell. The study also found that very few microRNAs are specific to individual tissues or cell types.Overall, the atlas quantifies microRNA expression in 26 types of normal and malignant tissues and cell types of humans and rodents. "They certainly have produced an extremely useful resource," said Victor Ambros of Dartmouth Medical School in Hanover, N.H., who was not involved in the work. They included "unprecedented diversity of tissue types in their survey and systematically characterized the microRNAs that are present there," he told The Scientist.MicroRNAs are small noncoding RNAs found in animals, plants, and viruses that regulate gene expression by interfering with messenger RNA function. Scientists have identified many microRNAs in mammals, but the expression levels and specificities of most of these...

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