Prime editing

A gene-editing method that writes a specified new DNA sequence into a target site using a template it carries with it.

• A Cas9 that nicks only one strand is fused to reverse transcriptase, an enzyme that copies RNA into DNA.
• Its guide RNA both finds the site and carries the replacement text, which is copied straight into the nicked strand.
• It can make all 12 possible single-letter swaps plus small insertions and deletions, a much wider range than base editing.
• The machinery is large and hard to package for delivery, and efficiency varies; the first human trial results arrived in 2025.