The antibiotic resistance gene of R plasmids which encodes for ampicillin resistance resides upon a 3.7 x 3700000 dalton sequence of DNA, TnA, which can migrate (translocate) from plasmid to plasmid. The translocation of TnA occurs independently of normal bacterial rec functions so that it permits rearrangement and recombination of DNA species from diverse backgrounds. The insertion of TnA is mutagenic and may have either a polar effect or a promotor effect depending upon its orientation of insertion. Translocation sequences such as TnA are powerful tools for genetic research since insertion of a resistance gene into a DNA molecule provides a readily identifiable genetic and physical marker. This feature has been exploited to mark Ent and K plasmids and can be used for the construction of vaccine strains. The epidemiological implications of the discovery that antibiotic resistance genes can migrate from molecule to molecule are most significant. No longer should one simply think of the direct extension of drug resistance plasmids by transfer but rather of an enormous reservoir of genes available to insert into any DNA species.