Post-transcriptional gene silencing(PTGS) is a defense mechanism of plants against foreign nucleic acids, such as virus infection. The mechanism results in sequence-specific degradation of nucleic acids, including endogenous mRNA, transgene mRNA and virus RNA. PTGS was first discovered in transgenic plants, and since then, similar mechanism has been found in fungi and animals. It appears that PTGS is initiated by aberrant RNA and double-stranded RNA in the cell. An enzyme similar to RNA-dependent RNA polymerase has been identified in various plants, which plays a key role in the PTGS process. It is hypothesized that PTGS might be the natural mechanism of plants against virus infection. To support this hypothesis, scientists from several laboratories have discovered PTGS suppressors encoded by virus genomes, and the suppressors identified so far are all viral pathogenicity determinants, such as viral movement protein.