Protospacer adjacent motif

Upon subsequent invasion, a CRISPR-associated nuclease such as Cas9 attaches to a tracrRNA–crRNA complex, which guides Cas9 to the invading protospacer sequence.

[11] Attempts have been made to engineer Cas9s to recognize different PAMs in order to improve the ability of CRISPR-Cas9 to edit genes at any desired genome location.

[17] CRISPR/Cas13a (formerly C2c2[18]) from the bacterium Leptotrichia shahii is an RNA-guided CRISPR system that targets sequences in RNA rather than DNA.

[19] A technology called GUIDE-Seq has been devised to assay off-target cleavages produced by such gene editing.

[20] The PAM requirement can be exploited to specifically target single-nucleotide heterozygous mutations while exerting no aberrant effects on wild-type alleles.