The role of viral oncoprotein expression in the maintenance of cellular transformation was examined as a function of time through controlled expression of simian virus 40 T antigen (TAg), Expression of TAg in the submandibular gland of transgenic mice from the time of birth induced cellular transformation and extensive ductal hyperplasia by 4 months of age, The hyperplasia was reversed when TAg expression was silenced for 3 weeks, When TAg expression was silenced after 7 months, however, the hyperplasia persisted even though TAg was absent, Although the polyploidy of ductal cells could be reversed at 4 months of age, cells at 7 months of age remained polyploid even in the absence of TAg, These results support a model of time-dependent multistep tumorigenesis, in which virally transformed cells eventually lose their dependence on the viral oncoprotein for maintenance of the transformed state.