As RNA guided endonuclease (Jinek et al., 2012).

As early as 1987, Ishino et al.
first discovered short
repeats in the genome of  E. coli K12, which is now recognized as spacer (Ishino et al., 1987). Later, researchers found that the similar interspaced
repeats also exists widely in the genomes of bacteria and the archaea (Mojica et al., 2000).

Later, Jansen et al. performed excessive studies on
the short repetitive DNA sequences in prokaryotes, characterized the CRISPR locus and coined the term as the clustered regularly
interspaced short palindromic repeats (CRISPR). They also identified
four CRISPR-associated (Cas) genes, suggesting that CRISPR/Cas system might
play a significant role in the  biological processes (Jansen et al., 2002; Mojica et al., 2000).

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In 2005, the association
 of these 
element to the immune system against the 
invading DNA was revealed (Bolotin et al., 2005; Mojica et al., 2005; Pourcel et al., 2005). 


In 2007, Barrangou et al.
determined that CRISPR-induced immunity was used to safeguard the bacteria from
invading phage (Barrangou et
al., 2007). In 2008, it found that the CRISPR system could hinder the
transfer of exogenous plasmid (Marraffini
et al. 2008). Overall, CRISPR loci have been reported to be found in
about 40% of bacterial and in most archaeal species with genomes sequenced.(Godde and Bickerton, 2006;
Bhaya et al., 2011).


These important
findings opened the new avenue to understand the functional  mechanism of CRISPR/Cas system for the
advancement of life sciences. In 2010, researchers suggested that spacer
sequences guide the Cas9 to cleave target DNA (Garneau et al., 2010). In 2011, it found that a duplex structure was generated
by tracrRNA and crRNA in associated with Cas9 (Deltcheva et al., 2011). In 2012, it was clearly
demonstrated that Cas9 was an RNA guided endonuclease (Jinek et al., 2012). In 2013, efficient and
specific genome editing was successfully achieved for the first time using
CRISPR/Cas9 in eukaryotic cells (Cong et al., 2013). In 2014, the crystal structure of Cas9 was
addressed and the interactions between Cas9 and gRNA and target DNA were
characterized (Nishimasu et
al., 2014). Recently, in 2015 a new gene editing system, CRISPR/Cpf1 was
found, a single RNA-guided endonuclease lacking tracrRNA. It utilizes a T-rich
protospacer-adjacent motif and cleaves DNA via a staggered DNA double-strand
break (DSB) (Zetsche et al.,