The organisms, such as knockout mice, are used to learn about a gene that has been sequenced, but whose function is unknown or incompletely known. Researchers draw inferences from the difference between the knockout organism and normal individuals. Knockout is often abbreviated as KO.
Gene knock-in is the opposite term. There a gene is switched on, or a working gene inserted.
If so, embryonic stem cells are genetically transformed and inserted into early embryos. Resulting animals with the genetic change in their germline cells can then often pass the gene knockout to future generations.