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 ribosome to translate the mRNA or by triggering degradation of the double stranded RNA by ribonuclease enzymes. Antisense RNA and DNA both have therapeutic potential for modifying the activity of disease causing genes.