Transgenesis is a sort of experiment in which a foreign gene is inserted into an organism's genome, passed down through the germ line, and the progeny's phenotypic is examined.
An exogenous gene of interest can be made to express itself by transgenesis.
Alternately, gene targeting can be used to disrupt (or "knockout") the sequence of a particular endogenous gene. the usage of mice, mostly transgenic and gene-targeted animals.
The intended outcome is for the transgenic organism to express the gene and display a novel trait.