A single transformant colony was isolated and viruses released from it were used to infect fresh cells, which where plated onto soft agar before new viruses could be released (1). The resulting colonies were amplified and the purified RNA was assayed for mutations by denaturing-gradient gel analysis. Several domains of known size were analyzed using RNA from 58 colonies, which was the equivalent of 65250 nucleotides (T_s = 3 ´ 65250 / 58 = 3375). Nine mutations were found, giving f = 9 / 65250 = 1.4 ´ 10^-4. Only one mutation was confirmed by sequencing. Assuming no superinfection, these mutations appeared during a single cell infection cycle. However, selection was probably present. Additionally, the number of provirus copies per cell was unknown and hence this estimate has to be taken with caution.

1. Leider, J. M., P. Palese, and F. I. Smith. 1988. Determination of the mutation rate of a retrovirus. J. Virol. 62:3084-3091.