Until today, gene transfer to germ cells has been attempted by a variety of methods including microinjection, embryonic stem cell-mediated transfection, virus mediated transfection, lipofection (1 ), microparticle bombardment (2 ), and sperm mediated transformation (3 ). In vivo gene electroporation (EP) now is a viable option for gene transfer purposes as demonstrated by strong shortterm gene expression and long-term expression after gene transfer (4 ,5 andunpublished). In vivo gene EP is simple and convenient. Adequate development and differentiation of transfected cells can be maintained in the in vivo environment. Furthermore, it can be applied, in principle, to any types of cells and tissues so long as the target is accessible. The advantages of in vivo EP over other nonviral methods such as lipofection and microparticle bombardment are: possible tissue damage is less; there is no limitation of DNA to be transfected at a time; and DNA can be transferable to cells deep inside the target tissue.
