Knock-out experiment is a method for deleting, mutating or otherwise inactivating the gene in a mouse. This laborious technique involves transfecting a crippled gene into cultured embryonic stem cells, searching through thousands of the resulting clones for one in which the crippled gene exactly replaced the normal one (by the homologous recombination), and inserting that cell back into the mouse blastocyst. The resulted mouse is chimaeric but, if you are lucky enough (and if you've gotten this far), its germ cells will carry the deleted gene. A few rounds of causious breeding can then produce progeny in which both the copies of the gene are inactivated.