The Cas9 endonuclease is the central component of the Type II CRISPR/Cas system, 
a prokaryotic adaptive restriction system against invading nucleic acids, such 
as those originating from bacteriophages and plasmids. Recently, this 
RNA-directed DNA endonuclease has been harnessed to target DNA sequences of 
interest. Here, we review the development of Cas9 as an important tool to not 
only edit the genomes of a number of different prokaryotic and eukaryotic 
species, but also as an efficient system for site-specific transcriptional 
repression or activation. Additionally, a specific Cas9 protein has been 
observed to target an RNA substrate, suggesting that Cas9 may have the ability 
to be programmed to target RNA as well. Cas proteins from other CRISPR/Cas 
subtypes may also be exploited in this regard. Thus, CRISPR/Cas systems 
represent an effective and versatile biotechnological tool, which will have 
significant impact on future advancements in genome engineering.