CRISPR, short for clustered regularly interspaced short palindromic repeats, was named for the specific repeated sequences of DNA in the bacterial genome. These sequences are a part of bacteria's immune system. Bacteria store viral DNA between the sequences. When a virus attacks the bacteria, this stored DNA is processed into a specialized RNA, crispr RNA or crRNA for short. The crRNA guides Cas9 protiens to seek out that specific sequence and cleave it. In bacteria, this allows them to destroy viral DNA. However by altering the targetting sequence in the crRNA, scientists can use CRISPR to cleave specific sequences of DNA and then insert new strands, thus precisely editing the genome.