The CRISPR pathway was discovered in bacteria, where it functions much like an immune system against invading viruses and plasmid DNA. Short DNA sequences (spacers) from invading viruses are incorporated at CRISPR loci within the bacterial genome and serve as “memory” of previous infection. Re-infection triggers the complementary mature CRISPR RNA (crRNA) to find a matching sequence – which provides the CRISPR-associated (Cas) nuclease the specificity to form a double-strand break at specific “foreign” DNA sequences.