Cas9_PI is a family found at the C-terminal of bacterial type II CRISPR system Cas9 endonuclease. This domain adopts a novel protein fold that is unique to the Cas9 family. It is positioned in the structure-DNA-complex to recognise the PAM sequence on the non-complementary DNA strand of the crRNA. PAM sequence is protospacer-adjacent motifs on DNA. See family CRISPR-DR2, Rfam:RF01315. Cas9 carries two nuclease domains, HNH and RuvC, which cleave the DNA strands that are complementary and non-complementary to the 20 nucleotide guide sequence in crRNAs, respectively [1].

1: Cut site selection by the two nuclease domains of the Cas9 RNA-guided endonuclease. Chen H, Choi J, Bailey S; J Biol Chem. 2014;289:13284-13294. PMID:24634220