prime editing
Base editing is gentle but limited — it only swaps certain single letters. Standard CRISPR is versatile but messy — it relies on double-strand breaks and inefficient repair. Prime editing was designed to combine the best of both: to write many kinds of precise change — single-letter swaps, small insertions, small deletions — directly into the DNA, without making a clean double-strand break and without needing a separate donor template. Its inventors nicknamed it a 'search-and-replace' tool for the genome.
Prime editing uses a souped-up guide and a fusion protein with two jobs. The guide RNA is special — called a pegRNA — because it both tells the editor where to go and carries the new sequence to write, like a sticky note with both the address and the message. The protein is a nicking Cas9 (it cuts only one strand, not both) joined to a reverse transcriptase, the enzyme that builds DNA from an RNA template. In plain steps: the pegRNA guides the editor to the target; the Cas9 nicks one strand; the freed strand end is used by the reverse transcriptase as a primer to copy the new sequence written in the pegRNA directly into the DNA; then the cell tidies up, and the second strand is updated to match. The edit is spelled out by the RNA you supplied.
Prime editing is powerful because it can in principle install most small edits without depending on the cell's hard-to-control double-strand-break repair, sidestepping both the messy indels of standard CRISPR and the narrow letter menu of base editors. It also tends to produce fewer off-target and unwanted byproduct edits. The honest caveats: it is mechanistically complex, often less efficient than the simpler tools, harder to design (the pegRNA has several parts that must work together), and still being refined — and like all editors it must be delivered into the right cells, which remains a major hurdle for any therapy.
To insert three missing letters at a precise spot, a pegRNA carries both the address and the three letters to add; the nicking Cas9 and reverse transcriptase write them in directly, no donor DNA and no double-strand break needed.
The RNA carries both the destination and the text to write.
Prime editing is often called the most versatile editor, and in principle it is — but versatility comes at the cost of complexity and, frequently, lower efficiency than base editing or standard CRISPR. It is not yet a finished, plug-and-play cure.