gene knock-in

A gene knock-in is the opposite of a knockout: instead of breaking a gene, you write something new into a precise spot. It is like editing a document not by deleting a word but by inserting a chosen phrase at exactly the right place — a tag, a corrected sequence, or a whole new gene.

Technically, a knock-in inserts a defined DNA sequence at a chosen genomic location. The usual recipe is to make a targeted cut (often with CRISPR-Cas9), then supply a repair template that carries the new sequence flanked by stretches matching the surrounding DNA. The cell's homology-directed repair copies the template in, leaving the insert seamlessly placed at the target.

Knock-ins enable many goals: correcting a disease-causing mutation back to the normal sequence, attaching a fluorescent tag to study where a protein goes, or adding a useful gene to a crop or cell line. They are generally harder and less efficient than knockouts, because precise template-based repair competes with the cell's faster, sloppier end-joining.