The bacteria and archaea adaptive immune systems clustered regularly interspaced short palindromic repeats (CRISPR)/Cas have been modified and engineered to efficiently, conveniently and precisely edit genomes in various cells and species both in and ex vivo. The gene editing toolkit is composed of two elements: the CRISPR associated protein (Cas9) and the guide RNA (gRNA). The guide RNA together with the genome matching sequence locate Cas9 nuclease to the target site demarcated by conserved sequences called proto-spacer adjacent motifs (PAMs). Under physiological conditions in bacteria and archaea, the exogenous phage genomes or plasmids that contain the target DNA sequences are cleaved and subjected to degradation. While the engineered systems  create site-specific double-stranded DNA breaks, which are then repaired either by homologous recombination or by non-homologous end joining.