CRISPR/Cas systems mediate bacterial adaptive immune responses that evolved to 
protect bacteria from bacteriophage and other horizontally transmitted genetic 
elements. Several CRISPR/Cas systems exist but the simplest variant, referred to 
as Type II, has a single effector DNA endonuclease, called Cas9, which is guided 
to its viral DNA target by two small RNAs, the crRNA and the tracrRNA. Initial 
efforts to adapt the CRISPR/Cas system for DNA editing in mammalian cells, which 
focused on the Cas9 protein from Streptococcus pyogenes (Spy), demonstrated that 
Spy Cas9 can be directed to DNA targets in mammalian cells by tracrRNA:crRNA 
fusion transcripts called single guide RNAs (sgRNA). Upon binding, Cas9 induces 
DNA cleavage leading to mutagenesis as a result of error prone non-homologous 
end joining (NHEJ). Recently, the Spy Cas9 system has been adapted for high 
throughput screening of genes in human cells for their relevance to a particular 
phenotype and, more generally, for the targeted inactivation of specific genes, 
in cell lines and in vivo in a number of model organisms. The latter aim seems 
likely to be greatly enhanced by the recent development of Cas9 proteins from 
bacterial species such as Neisseria meningitidis and Staphyloccus aureus that 
are small enough to be expressed using adeno-associated (AAV)-based vectors that 
can be readily prepared at very high titers. The evolving Cas9-based DNA editing 
systems therefore appear likely to not only impact virology by allowing 
researchers to screen for human genes that affect the replication of pathogenic 
human viruses of all types but also to derive clonal human cell lines that lack 
individual gene products that either facilitate or restrict viral replication. 
Moreover, high titer AAV-based vectors offer the possibility of directly 
targeting DNA viruses that infect discrete sites in the human body, such as 
herpes simplex virus and hepatitis B virus, with the hope that the entire 
population of viral DNA genomes might be destroyed. In conclusion, we believe 
that the continued rapid evolution of CRISPR/Cas technology will soon have a 
major, possibly revolutionary, impact on the field of virology.