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We have carefully examined the frequency of guanidine-resistant revertants in six different clonal pools of guanidine-dependent mutants of type 1 poliovirus. The mutation frequency was (6.5 +/- 6.3) x 10(-4) (with all amino acid substitutions occurring at position 227). The minimal corrected base substitution frequency per single nucleotide site in the codon for amino acid 227 was (2.1 +/- 1.9) x 10(-4).
Torre et al. (1990) studied this question.