CRISPR-Cas9 is derived from the immune system of prokaryotes, and knocking out genes at the genomic level can eliminate the expression of the target gene in the cell, while the function of the target protein may be completely lost. CRISPR-Cas9 contains two important components, a Cas9 protein that performs the double-stranded DNA cleavage function, and a gRNA (guide RNA) that has a directing function.  gRNA can guide Cas9 protein to a genomic target-specific sequence location for gene editing, and its target selection range can in principle cover all genomic sequences.