to a toxic compound (negative selection). Common examples Examples of selectable markers include: Beta-lactamase which confers ampicillin resistance to bacterial hosts. Neo gene from Tn5, which confers resistance to kanamycin in bacteria and geneticin in eukaryotic cells Mutant FabI gene (mFabI) from E. coli genome, which confers triclosan resistance to the host. URA3, an orotidine-5' phosphate decarboxylase from yeast is a positive and negative selectable marker. It is required for uracil biosynthesis and can complement ura3 mutants that are auxotrophic for uracil (positive selection). The enzyme URA3 also converts 5-fluoroorotic acid (5FOA) into the toxic compound 5-fluorouracil, so any cells carrying the URA3 gene will be killed in the presence of 5FOA (negative selection). Future developments In the future alternative marker technologies will need to be used more often to, at the least, assuage concerns about their persistence into the final product. It is also possible that markers will be replaced entirely by future techniques which use removable markers, and others which do not use markers at all, instead relying on co-transformation, homologous recombination, and recombinase-mediated excision. References Genetics techniques Molecular biology Antimicrobial resistance