The viral progeny released by a single transformant colony was used to infect fresh cells at a low moi, which were plated onto solid medium before the release of new viral progeny (1). The resulting infected colonies were analyzed by T1-RNAase digestion, covering a target of 1380 nucleotides (T_s = 3 ´ 1380 = 4140). Three substitutions were detected and confirmed by sequencing after screening ca. 151000 nucleotides in total, giving 3f_s / c / T_s = 3 ´ 3 / (3 ´ 151000) = 2.0 ´ 10^-5. However, selection was not completely absent. Since lethal or strongly deleterious mutations were probably missed, this should be considered as a lower-limit estimate. The selection correction factor given by the exponential plus lethal class model using c = 1, p_L = 0.3, E(s_v) = 0.12, selective sampling and taking B = 50 from the literature (2), is a = 0.48. Therefore, m_s/n/c =2.0 ´ 10^-5 / 0.48 = 4.2 ´ 10^-5.

1. Monk, R. J., F. G. Malik, D. Stokesberry, and L. H. Evans. 1992. Direct determination of the point mutation rate of a murine retrovirus. J. Virol. 66:3683-3689.

2. Niwa, O., A. Decleve, M. Liberman, and H. S. Kaplan. 1973. Adaptation of plaque assay methods to the in vitro quantitation of the radiation leukemia virus. J. Virol. 12:68-73.