Germ line transformation of mammals by pronuclear microinjection

The most popular approach for generating transgenic mammals is the direct i njection of transgenes into one pronucleus of a fertilized oocyte. In the p ast 15 years microinjection has been successfully applied in laboratory as well as in farm animals. The frequency of transgenic founders, although hig hly different between the species, is efficient enough to render this techn ique applicable to a wide range of mammals. The expression levels and patte rns of a transgene are initially influenced by the construction of the tran sgene. However, the overall phenotype of a transgenic organism is influence d by several genetic and environmental factors. Due to the Features of this technique not all of the genetic factors can be experimentally controlled by the scientist. In this article we will emphasize some peculiarities whic h have to be taken into account for the successful performance of transgene sis by pronuclear microinjection.