It is possible to generate tetraploidy in mammalian embryos by chemical or physical suppression of a cleavage division causing endoreduplication of the genome (1 ,2 ), or by using techniques to fuse karyoblasts or cells with nucleated or enucleated eggs and blastomeres, which results in transfer of nuclei or production of tetraploid embryos. Treatments with inactivated Sendai virus or with polyethylene glycol are effective in this regard (3 –5 ), but the use of electrofusion and its specific modifications for mammalian embryos has greatly facilitated the procedure (6 –9 ).
