The reversion frequency of an amber mutant was measured by plating the virus in permissive and selective cells (1). Only substitution UAG to UCG was viable (T = T_s = 1), and selection against the amber mutant was strong. Despite selection, it is still possible to use the null-class method to estimate the mutation rate. Out of 27 independent cultures assayed, 23 showed no revertants (P₀ = 23/27). The average number of pfu per culture was 1.8 ´ 10⁴ but the plating efficiency of the amber mutant was approximately three times lower than that of the revertant, so the corrected number is 5.4 ´ 10⁴ pfu. Hence, using Equation 10, m_s/n/r = - 3 ´ log (23 / 27) / (5.4 ´ 10⁴) = 8.9 ´ 10^-6.

1. Sedivy, J. M., J. P. Capone, U. L. RajBhandary, and P. A. Sharp. 1987. An inducible mammalian amber suppressor: propagation of a poliovirus mutant. Cell 50:379-389.