Single plaques were isolated and re-plated in the presence of anti-haemagglutinin monoclonal antibody (1). The mutation rate was estimated using the null-class method. Sequencing of a large number of clones (2) indicated that T_s = 2 and T_s = 5 for each of the two antibodies used. The mutation rates to the resistant phenotype were converted into mutation rates per nucleotide accordingly, yielding m_s/n/r = 3.0 ´ 10^-5 and m_s/n/r = 2.7 ´ 10^-6, with geometric mean m_s/n/r = 9.0 ´ 10^-6.

1. Suárez, P., J. Valcárcel, and J. Ortín. 1992. Heterogeneity of the mutation rates of influenza A viruses: isolation of mutator mutants. J. Virol. 66:2491-2494.

2. Suárez-López, P. and J. Ortín. 1994. An estimation of the nucleotide substitution rate at defined positions in the influenza virus haemagglutinin gene. J. Gen. Virol. 75:389-393.