A retroviral vector containing a neo gene with an amber codon (a neutral mutational target) and the hygro gene and was used to score mutations appearing during a single cell infection cycle by selecting clones with G418 resistance (cells containing proviruses revertant to the functional neo gene) and hygromycin (all provirus-containing cells) (1). The amber reversion frequency per cycle was f / c = 2.2 ´ 10^-5. It was determined that 15 / 17 revertants were to the wild-type, whereas the other two were a four-nucleotide insertion and an unidentified mutation. Using reversions to the wild type only, m_s/n/c = 3 ´ 2.2 ´ 10^-5 ´ 15 / 17 = 5.8 ´ 10^-5.

1. Dougherty, J. P. and H. M. Temin. 1988. Determination of the rate of base-pair substitution and insertion mutations in retrovirus replication. J. Virol. 62:2817-2822.