Knockout mouse
![A laboratory mouse in which a gene affecting hair growth has been knocked out (left), is shown next to a normal lab mouse.](/Images/godic/202501/22/Knockout_Mice5006-3002048.jpg")
![A knockout mouse (left) that is a model for obesity, compared with a normal mouse.](/Images/godic/202501/22/Knockoutmouse80-722048.jpg")
![The procedure for making mixed-genotype blastocyst.](/Images/godic/202501/22/Knockout_mouse_production_2.svg2048.png")
![Breeding scheme for producing knockout mice. Blastocysts containing cells, that are both wildtype and knockout cells, are injected into the uterus of a foster mother. This produces offspring that are either wildtype and coloured the same colour as the blastocyst donor (grey) or chimera (mixed) and partially knocked out. The chimera mice are crossed with a normal wildtype mouse (grey). This produces offspring that are either white and heterozygous for the knocked out gene or grey and wildtype. White heterozygous mice can subsequently be crossed to produce mice that are homozygous for the knocked out gene.](/Images/godic/202501/22/Knockout_mouse_breeding_scheme.svg2049.png")
A knockout mouse or knock-out mouse is a genetically modified mouse (Mus musculus) in which researchers have inactivated, or "knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. The loss of gene activity often causes changes in a mouse's phenotype, which includes appearance, behavior and other observable physical and biochemical characteristics.