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.
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