Polyoma middle T antigen requires cooperation from another gene to express the malignant phenotype in vivo.

The oncogenic potential of polyomavirus in newborn hamsters can be expressed by a recombinant encoding only the middle T protein. However, polyoma middle T requires the cooperation from small T to induce tumors in newborn rats. Similar complementary functions such as cocarcinogens or tumor promotors can be exerted by the simian virus 40 T antigens as well as by one or several products of the early region 1A of adenovirus 2.