This is a specific type of transgenesis that targets a particular gene. If a mutated copy of a gene is electroporated into a cell, the inserted DNA will find the endogenous copy of itself and recombination will occur with some frequency (1-25%). If this event occurs in embryonic stem cells, cells carrying the new copy of the gene can be used to generate embryos that can be assessed for the phenotypic consequences of the mutation. This technique is used frequently in mice to study loss-of -function mutations. Read more about gene targeting references: Thomas et al. High frequency targeting of genes to specific sites in the mammalian genome. Cell 1986; 44(3):419-28. van der Weyden L, et al. Tools for targeted manipulation of the mouse genome. Physiol Genomics 2002; 11(3):133-64.