The accidental discovery of the mouse polyoma virus nearly 50 years ago ope
ned up an experimental system unique in opportunities for investigating vir
us-host interactions leading to the development of tumors. Extensive studie
s of the virus in tissue culture have provided a detailed understanding of
its genetics and molecular biology. Knowledge of the virus as a transformin
g agent in culture can now be tested in the animal where multiple cell type
s are targets for tumorigenic conversion and where a variety of host factor
s, both immunological and nonimmunological, come into play. Studies in the
animal using well-characterized wild-type and mutant virus strains have led
to some unexpected findings. Some of these run counter to certain widely h
eld beliefs in cancer biology. This minireview focuses on these surprising
findings and the challenges they raise. (C) 2001 Academic Press.