%%CRISPR%%: short for clustered regularly interspaced short palindromic repeats. In bacteria, %%CRISPR%% is a form of innate immunity that bacterial cells use as a defense against bacteriophage. The %%CRISPR%% system consists of a protein typically called %%Cas9%% and two non-coding RNAs called %%gRNA%% and tracrRNA. The %%gRNA%% directs the %%CRISPR/Cas9%% enzyme to a DNA target based on base pairing between the %%gRNA%% sequence and the target DNA.

Scientists have co-opted the %%CRISPR/Cas9%% system as a research tool that can be used to cleave dsDNA at nearly any desired location with a high degree of specificity. They do this by modifying the %%gRNA%% sequence to match a desired target DNA.