Altogether, the proportion of recombinant genomes found in the mixed viral populations was astonishingly high and very similar in the ten co-infected plants analyzed (Table 1 , last column), ranging between 44% (plant 5) to 60% (plants 7, 12, and 20), with a mean frequency (6 standard error) of 53.8% 6 2.0%. This result indicates that recombination events are very frequent during the invasion of the host plant by CaMV and represents, to our knowledge, the first direct quantification of viral recombination during the infection of a live multi-cellular host.