Compelling evidence indicates that the CRISPR-Cas system protects prokaryotes 
from viruses and other potential genome invaders. This adaptive prokaryotic 
immune system arises from the clustered regularly interspaced short palindromic 
repeats (CRISPRs) found in prokaryotic genomes, which harbor short 
invader-derived sequences, and the CRISPR-associated (Cas) protein-coding genes. 
Here, we have identified a CRISPR-Cas effector complex that is comprised of 
small invader-targeting RNAs from the CRISPR loci (termed prokaryotic silencing 
(psi)RNAs) and the RAMP module (or Cmr) Cas proteins. The psiRNA-Cmr protein 
complexes cleave complementary target RNAs at a fixed distance from the 3' end 
of the integral psiRNAs. In Pyrococcus furiosus, psiRNAs occur in two size forms 
that share a common 5' sequence tag but have distinct 3' ends that direct 
cleavage of a given target RNA at two distinct sites. Our results indicate that 
prokaryotes possess a unique RNA silencing system that functions by 
homology-dependent cleavage of invader RNAs.