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1. A pair of genes that encode a stable, toxic protein and a corresponding unstable antitoxin that is either a protein or RNA, found in many bacteria either on a plasmid or on the bacterial chromosome.
2. A mixture of a toxin and its antitoxin with a slight excess of toxin, formerly used as a vaccine.
(Pharmacology) a mixture of a specific toxin and antitoxin. The diphtheria toxin-antitoxin was formerly used in the US for active immunization