Transgenic mice harboring the SV40 large T antigen gene in a C57B1/6J genetic background (SV11) first express this gene at 1-2 weeks of age, develop papillomas of the choroid plexus by 80-90 days, and die within 125 days after birth. Transgenic mice harboring the same transgene in a (SV11-C57Bl/6J x NZW/lacJ) F1 genetic background express considerably lower levels of the transgene mRNA at comparable times after birth. As a consequence, tumor development and death are delayed. The NZW mice appear to contribute a dominant negative regulator for the expression of the SV40 large T antigen transgene, which in turn has a dramatic effect upon the time of appearance of tumors and the death of these transgenic animals.