An RNA molecule whose base sequence is complementary to that of the RNA transcript of a gene, i.e. the ‘sense’ RNA, such as a messenger RNA (mRNA). Hence, an antisense RNA can undergo base pairing with its complementary mRNA sequence. This blocks gene expression, either by preventing access for ribosomes to translate the mRNA or by triggering degradation of the double-stranded RNA by ribonuclease enzymes. Antisense RNA has therapeutic potential for modifying the activity of disease-causing genes, and drugs based on this mechanism are being developed. Also, genes encoding antisense RNAs can be used in genetic engineering to alter the makeup of organisms. For example, the FlavrSavr tomato was engineered with an artificial gene for antisense RNA that prevented expression of a gene for an enzyme involved in ripening, in order to retard spoilage. In the 1980s it was discovered that double-stranded RNA molecules have a much greater ability to suppress their corresponding genes than single-stranded RNAs, due to the phenomenon of RNA interference.