The sgRNA is a hybrid of two components: CRISPR RNA (crRNA; blue) and trans-activating crRNA (tracrRNA; red). (A) Cleavage of the target DNA by the Cas9 nuclease results in mutations that can knock out the target gene. (B) If an appropriate donor DNA molecule is available, genetic information (shown here in pale blue) can be added to the target DNA in a precise manner. (C) By targeting two Cas9 nucleases to different regions of the target DNA, it is possible to delete the genetic information between the two regions. CRISPR: clustered regularly interspaced short palindromic repeats; Cas: CRISPR-associated.