Base editing

A gene-editing method that chemically converts one DNA letter into another at a targeted site without cutting through the DNA.

• A Cas9 disabled from cutting both strands is fused to an enzyme that rewrites one base: C to T, or A to G.
• Many disease mutations are single-letter changes of exactly these types; in 2025 a custom base editor was made in months to treat an infant's rare liver disease.
• With no double-strand break there are fewer large deletions, which suits permanent one-time treatments such as switching off a cholesterol gene.
• Only certain swaps work, identical letters near the target may also change, and delivering the editor into tissues remains hard.