Model of CRISR/Cas system function. (A) Basic structure of the CRISPR/Cas system: several Cas proteins are encoded by the cas genes, located typically in the close proximity of the CRISPR arrays, composed of a variable number of DNA repeats interrupted by unique spacer regions which originate from foreign DNA acquisition. (B) The Cas proteins are in charge of the crRNAs processing and interference with the cDNA region, promoting DNA degradation or silencing (C).