An experimental technique that disables a specific gene to study its function by observing the resulting phenotype.A gene knockout is a laboratory method for finding out what a particular gene actually does by removing it from an organism's DNA and watching what changes. Because a gene's job is usually inferred from the consequences of losing it, the knockout turns an abstract sequence of letters into an observable effect: a mouse that cannot smell, a yeast cell that stops growing on a certain sugar, a plant that wilts sooner. The approach rests on a simple logic — if you want to know what a part does, take it out and see what breaks. This makes knockouts one of the most direct tools in genetics, complementing methods that instead add or overexpress genes.
Targeted gene disruption by homologous recombination in mouse embryonic stem cells was developed in the 1980s, notably by Mario Capecchi, Martin Evans, and Oliver Smithies, work recognized with the 2007 Nobel Prize in Physiology or Medicine. Earlier, yeast geneticists had used similar replacement strategies to delete genes systematically.
In its classic form, researchers replace the target gene's coding sequence with a selectable marker, such as an antibiotic-resistance cassette, using homologous recombination: the cell's own DNA repair machinery swaps the engineered piece into the chromosome because it matches the flanking sequences. Cells that took up the replacement are then selected by growing them in the presence of the antibiotic. In organisms where homologous recombination is inefficient, related strategies are used — CRISPR-Cas9 cuts the gene and lets error-prone repair disable it, RNA interference reduces its output without altering the DNA, and conditional systems delete the gene only in chosen tissues or at chosen times. The resulting organism, cell line, or plant is then compared with an unmodified control to see which traits depend on the missing gene.
Knockout mice and knockout cell lines are standard in biomedical research for modeling human disease and for assigning functions to genes; large-scale knockout libraries in yeast and human cells are used to screen which genes a drug or pathogen depends on.
The first reported knockout mice, made in the late 1980s, carried a disabled gene for an enzyme used in purine metabolism, and the animals were viable — showing that not every gene is essential for life.
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