knockout mouse
A knockout mouse is a mouse in which one chosen gene has been deliberately switched off, so researchers can see what happens when that gene is missing. The logic is simple and powerful: if you want to know what a part does, remove it and observe what breaks. The absent gene reveals its normal job through the mouse's altered biology.
To make one, scientists inactivate the target gene — historically by replacing it in embryonic stem cells, now often with genome editing — and breed mice that lack a working copy. Comparing knockout mice with normal littermates shows which tissues, behaviours or developmental steps depend on the gene, making this a cornerstone of reverse genetics in mammals.
Results require careful reading. Some knockouts die before birth, revealing essential roles but cutting study short; others look surprisingly normal because related genes cover for the loss. Refinements such as switching a gene off only in certain tissues or at certain times help separate a gene's many roles.
To test whether a gene is needed for a working immune system, researchers knock it out in mice; if the knockout animals cannot fight off infection, the gene's role becomes clear.
Remove the gene, observe the consequence, infer the function.
A “conditional knockout” turns the gene off only in a chosen tissue or after a chosen time, which is invaluable when fully removing the gene would kill the embryo before the question of interest can be studied.