CRISPR

A gene-editing system that uses a programmable guide RNA to steer a DNA-cutting enzyme, most often Cas9, to a precise site.

• A guide sequence of about 20 letters matches the target DNA; the Cas enzyme binds there and cuts both strands of the double helix.
• The cell repairs the break, often imperfectly, which disables the gene; supplying a repair template can write in a precise change, but less efficiently.
• Casgevy, approved in 2023 for sickle-cell disease and beta-thalassaemia, was the first CRISPR medicine; it edits patients' own blood stem cells.
• Risks are cuts at look-alike 'off-target' sites and occasional large deletions or rearrangements where both strands break.