  The CRISPR/Cas9 system acts as the prokaryotic immune system and has
important applications in gene editing. The protein Cas9 is one of its crucial
components. The role of Cas9 is to search for specific target sequences on the
DNA and cleave them. In this Letter, we introduce a model of facilitated
diffusion for Cas9 and fit its parameters to single-molecule experiments. Our
model confirms that Cas9 search for targets by sliding, but shows that its
sliding length is rather short. We then investigate how Cas9 explores a long
stretch of DNA containing randomly placed targets. We solve this problem by
mapping it into the theory of Anderson localization in condensed matter
physics. Our theoretical approach rationalizes experimental evidences on the
distribution of Cas9 molecules along the DNA.
