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In 1929 by adding formaldehyde, Ramon demonstrated the conversion of diphtheria toxin to its nontoxic, but antigenic equivalent: the toxoid. In 1951, Freeman found that the pathogenic strains of C. diphtheriae are lysogenic. Toxin producers are infected by a temperate betta-phage. The gene for toxin production is located on the chromosome of the betta-phage.