Although mutations in codons 164 and 179 of poliovirus polypeptide 2C can establish guanidine resistance, the majority of guanidine-resistant and guanidine-dependent viruses contain an M → L change at amino residue 187 along with other critical mutations. The change at residue 187 probably contributes to the guanidine phenotype along with a variety of other modifications in polypeptide 2C, possibly altering conformation of the polypeptide during the initiation step of viral RNA synthesi