A knockout mouse is a mouse in which a specific gene has been deleted through a technique known (rather dauntingly) as targeted gene deletion by homologous recombination. Modern geneticists have been generating knockout mice for about a decade, and have used them to explore the function of many genes and also to create models of various human diseases.
The crucial idea underlying the generation of knockout mice is that the phenotype (physical appearance and behaviour) of the resulting mouse will shed light on the function of the deleted gene - rather like the way removing that removing your own lungs would rapidly demonstrate that the function of your lungs had something to do with breathing. Of course, it is important to interpret this information very carefully. Removing the antenna from your TV blocks the formation of meaningful images and generates static; this does not mean, however, that the function of the antenna is either "image formation" or "static prevention."
I have spent most of this year working on the creation of a knockout mouse, which will (hopefully) come to final fruition some time next year. As I write up my thesis over the next month, this node will be fleshed out with details of the process of genetically engineering a mutant rodent to further the progress of science; and as I continue my project next year, I will add information on the process of characterising the phenotype of my mutant mouse.
More to come...