 A year later, a study confirmed the presence of type II-A CRISPR–Cas9 inhibitors (AcrIIA1, AcrIIA2, AcrIIA3 and AcrIIA4) in "Listeria monocytogenes" (infected by bacteriophages which introduced the anti-CRISPR proteins).Two of those proteins (AcrIIA2 and AcrIIA4) were demonstrated to work properly against "Streptococcus pyogenes" type II-A defensive CRISPR system.The result of all this research has been the discovery of 21 different Anti-CRISPR protein families, despite other inhibitors may exist due to the quick mutational process of phages.