DNA-based Genetic Analysis and Biotechnology
For some years, it has been possible to create transgenic rodents w h i c h carry addit ional D N A sequences introduced through pronuclear injection. These have been very successful ly used to address the effects of overexpression of any chosen gene. However , this technology suffers three pr inc i - pal l imitat ions: first, there is no control over where the foreign D N A integrates into the host genome; second, there is no control over the number of copies of the introduced gene; and, t h i r d , this technology is l imited to the addi t ion of D N A , and cannot therefore be used to look at loss of gene funct ion. These l imitat ions stimulated efforts to create a more controllable methodology, w h i c h finally arose out of the dual technologies of embryonic stem (ES) cell culture and gene targeting. In combinat ion , these approaches n o w a l l o w the product ion of murine strains w h i c h bear precise genetic alterations, inc luding both the addi t ion and subtract ion of sequences. This approach has been termed "gene target ing" .