these are ( 1 ) only the has of virulent influenza viruses were cleaved in tissue culture in the absence of trypsin and ( 2 ) virulent h5 influenza viruses contain a series of basic amino acids at the cleavage site of the ha , whereas avirulent strains contain only a single arginine with the exception of the avirulent chicken/pennsylvania virus .
C0301714
basic amino acids
C0003765
arginine