CRISPR is a system bacteria evolved to defend against viruses. Researchers realised it could be directed at any sequence, which turned it into the most precise gene editing tool available.
The bacterial original
When a virus infects a bacterium and the bacterium survives, it stores a fragment of the viral DNA in its own genome as a record.
Later, it makes RNA copies of those fragments and attaches them to a protein that cuts DNA. The RNA acts as a guide – the protein moves along DNA until it finds a match, then cuts.
An immune memory implemented in molecules.
The repurposing
The guide RNA can be made to match any sequence. Supply your own and the protein cuts wherever you specify.
The cell then repairs the break. Repair is error-prone, which can disable a gene. Or you can supply a template, and the cell may use it – which lets you write a specific change.
What it can do
Disable a gene reliably. Insert or change a sequence, less reliably. Do both in cells outside the body with high precision, which is how the first approved therapies work.
What remains difficult
Delivery into the right cells inside a living body. This is the main obstacle to most applications.
Off-target cuts at sequences resembling the target. Newer variants have reduced this substantially.
And anything involving many genes at once, which is most complex traits.
The ethical line
Editing cells that do not pass changes to offspring is broadly accepted. Editing embryos is prohibited nearly everywhere, following a case that produced international condemnation.