Figure 1.

Application of the CRISPR/Cas9 System in precision genome editing. The mechanism of CRISPR/Cas9-mediated targeted mutagenesis is shown. The single guide RNA (sgRNA) forms a ribonucleoprotein complex with the Cas9 endonuclease and directs the enzyme to the target DNA. Following cleavage of the sequence, repair mechanisms (i.e., homology directed repair, HDR; non-homologous end joining, NHEJ) are activated to mend the DNA damage. This often results in the insertion or deletion (indel) of nucleotides, which may alter the reading frame of the gene through the introduction of pre-mature stop codons. If the encoded protein domain is functionally essential, these indels generate a high proportion of null mutations.