In a recent News and Views1, J. P. Stoye discussed the potential health risks associated with the spread of endogenous proviruses from pigs and primates through the use of their organs for transplantation into humans. Although the risks to patients and the public from horizontal transmission may be as manageable as he represents, we would like to comment on his reference to the presence of vertically transmitted proviruses in the genomes of all mammals as “mementos”.