Bacteria keep a file of the viruses that attacked them

A CRISPR array is a list, written in DNA, of snippets taken from viruses. The cell uses the list to recognise those viruses and cut them. The gene-editing tool is that immune system, borrowed.

Clustered repeats had been noticed in bacterial genomes for years before anyone knew what they were for. In 2007 Rodolphe Barrangou and colleagues showed the function in a yogurt culture of Streptococcus thermophilus. Bacteria that survived a phage carried new spacers whose sequence matched that phage, and those spacers made the next infection fail. Add the spacer and you add the resistance. Take it away and the resistance goes. The cell transcribes the list, loads each snippet into a cutting protein, and the protein cuts whatever matches.

What laboratories do with Cas9 is the same recognition, aimed by a guide we type instead of a guide the cell captured. That re-aiming is real and it is young, and it is not a cure sitting in a drawer. Off-target cuts are a measurement problem, delivery into a human organ is an engineering problem, and most diseases are not a single site waiting to be snipped. The fact underneath the industry is older and stranger. An immune system can be a paragraph of DNA, inherited, and edited by the infection itself.