{HEAD ~ Progressive mutation} of {TAIL ~ the virus} by such nucleoside analogs renders it non viable 
of
{ HEAD ~ Progressive mutation } of { TAIL ~ the virus } by such nucleoside analogs renders it non viable
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH