RNA interference (RNAi) takes place in the nuclei of human cells, according to a group from the University of Massachusetts Medical School. Tariq M. Rana, a coauthor on the study, says that until now researchers thought that human activated RNA-induced silencing complexes (RISCs) localized and functioned only in the cytoplasm where mature mRNA is translated. Rana and colleagues designed a 21-nucleotide small interfering RNA (siRNA) that targeted 7SK small nuclear RNA (snRNA), an abundant RNA with a highly defined structure that specifically localizes to the nucleus. Introducing the siRNA into cells resulted in a 94% decrease in snRNA levels, they found. "We learned that the nucleus contains activated RISC, which can cleave target RNA in vitro," says Rana.