The recent development of gene transfer methods for almost all eukaryotes has revealed that transgenes can undergo silencing after integration in the genome.  Host genes can also be silenced as a consequence of the presence of a homologous transgene, thus limiting the potential application of genetic transformation.  Despite this limitation, transgene-induced gene silencing events were considered originally as anecdotal phenomena.  However, as more and more similarities were found between transgene-induced gene silencing and natural epigenetic phenomena, considerable interest has been devoted to this subject.