The clustered regularly interspaced short palindromic repeats (CRISPR)-associated endonuclease Cas9 seems one of the most promising approach of genetic engineering to treat a number of diseases, including HIV infection.

A recent study at the CINIMA -University of Amsterdam, The Netherlands- has demonstrated that CRISP can strongly inhibit HIV viral replication, but the virus rapidly escape from this inhibition, by generating resistant variants. The CRISP approach is potent and highly selective: it can be used for guide RNA (gRNA)-directed, sequence-specific cleavage of HIV pro- viral DNA in infected cells.

As already noticed for other so specific therapeutic approaches, a long term treatment is poorly effective. Viruses develop resistance mechanisms to survive and continue the infection. In this case, HIV virus uses a proofreading system of mammalian cells.  In these cells, the double-stranded breaks can be repaired by the non-homologous end-joining (NHEJ) pathway, which frequently introduces insertions, deletions, and nucleotide substitutions at the cleavage site, or by homology-directed repair, which depends on the presence of homologous DNA sequences. These mutations in the target sequence of Cas9 avoid the identification of the cleavage site and ultimately make ineffective the CRISP approach. Therefore, it is important to keep in mind this point when such as therapeutic systems are proposed for highly replicating cells and viruses.