 AcrIIA4 is one of the proteins responsible for the CRISPR-Cas9 system inhibition, the mechanism used in mammalian cells edition.Addition of AcrIIA4 in human cells avoids Cas9 interaction with the CRISPR system, reducing its ability to cut DNA.However, diverse studies have reached the conclusion that adding it in small proportions after the genome editing has been done, reduces the number of off-target cuts at the concrete sites in which Cas9 interacts, a thing that makes the whole system much more precise.