Anti-CRISPR (Anti-Clustered Regularly Interspaced Short Palindromic Repeats or Acr) is a group of proteins found in phages, that inhibit the normal activity of CRISPR-Cas, the immune system of certain bacteria. CRISPR consists of genomic sequences that can be found in prokaryotic organisms, that come from bacteriophages that infected the bacteria beforehand, and are used to defend the cell from further viral attacks. Anti-CRISPR results from an evolutionary process occurred in phages in order to avoid having their genomes destroyed by the prokaryotic cells that they will infect.