The questions raised by Dulbecco (1975)--"Is transformation the result of provirus integrat ion.., or is it the consequence of the expression of genes present in the provirus?"--have not yet been satis-factorily answered. However, several lines of evidence from recent experiments on many different tumor viruses, as presented in this volume, strongly suggest that viral gene expression is required for transfor-mation, the best evidence still arising from studies on the avian RNA tumor viruses (see Vogt 1977; Hanafusa 1977). With regard to polyoma virus, evi-dence also exists which indicates that viral gene expres-sion is needed. Studies by Seif and Cuzin (1977) on transformation i duced in rat cells by temperature-sensitive tsA mutants of polyoma virus suggest hat
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Griffin et al. (1980) studied this question.