definiens: . These sitcssdefine to points in the gene at whicb a deletion ( Icnockout ) is desired . A gene tbat has been targeted with Lox P sites is said to be A secood component of this method requires the participation of a second mouse strain, one in which a foreign gene encoding an enzyme called bas been introduced ; This enzyme bas the unique ability to digest Lox P elements in DNA, thereby inactivating the Floxed gene . If the foreign CRE gene is uptessed in only ccrtsin tissues in the mouse, or at only certain times of development, or only when a particular drug is given, the Imocking - out of the Floxed gene can occur in a specific, pre - ordained way . Mouse strains that bave genes are perfcctly normal until the eRE acts . Thus, for instance, adult - disease models, or acute liver~ models can be cruted . These animal models are critical for next - generation genetic undemanding of environmental influences upon human health