An antiviral Dicer defends stem cells Stem cells have a pivotal role in maintaining tissue architecture, integrity, and renewal. Poirier et al. demonstrate that mammalian stem cells can protect themselves from some RNA viruses by expressing an alternatively spliced isoform of the enzyme Dicer called aviD, which potentiates antiviral RNA interference (see the Perspective by Shahrudin and Ding). aviD acts by cleaving long, base-paired viral RNAs to generate small interfering RNAs that direct the sequence-specific cleavage of homologous viral RNAs. This process is reminiscent of that in insects and worms, which also use Dicer-dependent RNA interference in antiviral defense, and contrasts with mammalian differentiated cells, which generally rely on the interferon system to combat virus infection. Science , abg2264, this issue p. 231 ; see also abj5673, p. 160