In our laboratory, the first target for zwitter-ionic pronucleotides were glioblastoma cells that are known to overexpress thymidylate synthase (TS) (Grunda et al., 2006), and thus as a nucleoside part 2′-deoxy-5-fluorouridine (FdU) was chosen since its 5′-phosphate is an effective inhibitor of this enzyme (Hartmann and Heidelberger, 1961). The gene discussed is TYMS; the disease is glioblastoma.