The CRISPR/Cas system in prokaryotes provides resistance against invading 
viruses and plasmids. Three distinct stages in the mechanism can be recognized. 
Initially, fragments of invader DNA are integrated as new spacers into the 
repetitive CRISPR locus. Subsequently, the CRISPR is transcribed and the 
transcript is cleaved by a Cas protein within the repeats, generating short RNAs 
(crRNAs) that contain the spacer sequence. Finally, crRNAs guide the Cas protein 
machinery to a complementary invader target, either DNA or RNA, resulting in 
inhibition of virus or plasmid proliferation. In this article, we discuss our 
current understanding of this fascinating adaptive and heritable defense system, 
and describe functional similarities and differences with RNAi in eukaryotes.