knockout A technique for inactivating a particular gene or genes within an organism or cell. Both single-celled and multicellular organisms can be genetically engineered so that a normal gene is replaced with a defective homologous gene. Experiments with laboratory organisms (especially mice) treated in this way reveal how defects in particular genes can affect the development and life of the organism. Knockout mice are engineered by injecting normal mouse oocytes with defective DNA and selecting transfected embryos that are heterozygous for the defective gene. If adults reared from such embryos are crossed with other similar heterozygotes, 25% of the progeny will be homozygous for the ‘knocked out’ gene, and hence the impact of the defect on such mice can be assessed. A less laborious alternative to knockout uses the phenomenon of RNA interference to suppress the expression of specific genes in cultured tissue cells – so-called gene silencing. The cells are transfected with small double-stranded RNA molecules, tailor-made to bind to specific RNA transcripts inside the cell nuclei, thereby causing degradation of the transcripts or preventing their translation into protein.

knock·out / ˈnäkˌout/ • n. an act of knocking someone out, esp. in boxing: [as adj.] a knockout blow. ∎ inf. an extremely attractive or impressive person or thing: he must have been a knockout when he was young.