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Tr''aat is the role of the chemical?:-- We believe that the chemical accelerates the rate of transformation by blocking or derepressing the host cell gene control mechanisms. Thus, chemical carcinogens alone would increase the rate of transforma- tion by der ess enous ,~ral oncoger.es a:1 :.m a few cases, synthesis of infectious virus may also be derepressed ( Tdhy do chemicals and viruses enhance each other as ` a::sforminz agents?:--in chronically infected cells, multiple copies of the oncogene are produced, but cannot usually induce tra,-isforr..ation due to the cell's repressor mechanisms. 'Under these conditions, inhibition of the cell's repressor mechanisms results in a h igh rate of transformation. G onclusion In conclusion we must make two final observations. First, although it is the virus which provided the oncogenic information, the chemicals are extremely important since they change the order of magnitude of the transformation rate. This means that events which would be seen but rarely in the lifeline of the animal would occur with higher frequency upon prolonged exposure to carcinogens. Secondly, regardless of the validity of our interpretation of the relative roles of chemical carcinogens and viruses, it seems probable that there is no such thing as a cell culture completely free of viral genetic information { } and those engaged with in vitro studies must take this into account.

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