antisense, DNA or RNA manipulated in a laboratory so that its components (nucleotides) form a complementary copy of normal, or "sense," messenger RNA (mRNA; see nucleic acid). Antisense techniques are used to deactivate disease-causing or undesirable genes so that they cannot produce harmful or unwanted proteins. (Conventional drugs bind directly with disease-causing protein molecules, but their imperfect specificity may lead them to bind with other protein molecules, resulting in unwanted side effects. Antisense molecules are extremely specific.) In some applications of this technique, the antisense nucleic acid segment is inserted into an inactivated or nonvirulent virus, then introduced into the cell. The antisense segment pairs with the mRNA, preventing the synthesis of protein by the mRNA. Antisense has applications in agricultural biotechnology, where it has been used to deactivate the gene that causes softening in tomatoes, and in medicine, especially in cancer and antiviral therapy. See also gene therapy.