A gene knockout is a genetically engineered organism that carries one or more genes in its chromosomes that have been made inoperative (have been "knocked out" of the organism). This is done for research purposes. Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function. Researchers draw inferences from the difference between the knockout organism and normal individuals.
The term also refers to the process of creating such an organism, as in "knocking out" a gene.
Knockout is accomplished through a combination of techniques, beginning in the test tube with a plasmid, a bacterial artificial chromosome or other DNA construct, and proceeding to cell culture. Individual cells are genetically transformed with the construct and--for knockouts in multi-cellular organisms--ultimately fused with a stem cell from a nascent embryo.
The construct is engineered to recombine with the target gene, which is accomplished by incorporating sequences from the gene itself into the construct. Recombination then occurs in the region of that sequence within the gene, resulting in the insertion of a foreign sequence to disrupt the gene. With its sequence interrupted, the altered gene in most cases will be translated into a nonfunctional protein, if it is translated at all.
A conditional knockout allows gene deletion in a tissue no terminal manner.
Because recombination is a rare event in the case of most cells and most constructs, the foreign sequence chosen for insertion usually includes a reporter. This enables easy selection of cells or individuals in which knockout was successful.
In diploid organisms, which contain two alleles for most genes, and may as well contain several related genes that collaborate in the same role, additional rounds of transformation and selection are performed until every targeted gene is knocked out. Selective breeding may be required to produce homozygous knockout animals.
Knock-in is similar to knock-out, but instead it replaces a gene with another instead of deleting it. To replace the gene unit, one must use the chemical imulsion. Knock-in is also used to rejuvenate downed COGs in order to resume the functionality of the gene unit.