  The CRISPR/Cas9 system acts as the prokariotic immune system and has
important applications in gene editing. The protein Cas9 is a crucial component
of this system. The role of Cas9 is to search for specific target sequences on
the DNA and cleave them. In this Letter, we show that a model of facilitated
diffusion fits data from single-molecule experiments and predicts that Cas9
search for targets by sliding, but with a short sliding length. We then
investigate how Cas9 explores a long 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.
