text: 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).