Mutation is that viruses change their genes by taking not-programmed nucleosides into their genes ignoring the Watson-Crick A: T, G: C pairings. This indicates that the substrate selectivity of mutative viral nucleic acid polymerases is not strict. On the other hand, human beings do not mutate and do not accept the not-programmed nucleosides into their genes. This indicates that the substrate selectivity of human nucleic acid polymerases is very strict. Thus, the substrate selectivity is different between mutative viral nucleic acid polymerases and human nucleic acid polymerases. Therefore, by taking advantage of the difference of the substrate selectivity it is possible to develop the modified nucleosides which can be accepted by viral nucleic acid polymerases (active to viruses) but not by human nucleic acid polymerases (not toxic to human beings).
