Fouremm,

Most applications of CRISPR are for genome editing; that is permanent changes to the DNA......unless one repeats the process with a re-designed CRISPR intervention to change the DNA again....either back to original sequence or a new one. However, one component of this CRISPR system is the Cas9 nuclease. There are certian versions of the Cas9 that are deficient in nuclease activity. This allows one to achieve sequence specific gene silencing or activation via specific DNA-binding, but the lack of nuclease activity prevents DNA cleavage and thus there is no permanent DNA alteration. So this latter applicaition of CRISPR with the nuclease deficient Cas9 can be viewed as somewhat reversible.

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