The N1 NA of pH1N1 that carried the H275Y mutation showed an 300-fold increase in oseltamivir IC 50 values (Table 1 ), but the mutation did not confer resistance to zanamivir. There was a 1.3-fold decrease in the binding affinity of the enzyme for the small MuNANA substrate resulting from the resistance mutation as indicated by the difference in K m values (Table 1) . Furthermore, the EC 50 of oseltamivir for virus containing wild-type NA was between 10 and 100 nM, whereas for the H275Y mutant this value was .10 mM since the virus still formed plaques at this concentration of drug (data not shown).