Abstract
Mercury resistance is found in many genera of bacteria. Common amongst enterobacteria are transposons related to Tn21, which is both mercuric ion- and streptomycin-/spectinomycin- and sulphonamide-resistant. Other Tn21-related transposons often have different antibiotic resistances compared with Tn21, but share many non-antibiotic-resistance genes with it. In this article we discuss possible mechanisms for the evolution of Tn21 and related genetic elements.