BEFORE ANIMAL cells were experimentally converted to malignancy by certain viruses, there was no evidence of continuous transmission of noninfectious, viral genetic material in them. The recognized manifestations of viral infection in animal cells were associated with synthesis of infectious virus which led either to destruction of the infected cells or to the cellular proliferation seen in the benign papillomas of viral etiology. The few, naturally-occurring, malignant tumors of established viral etiology are caused by viruses (apparently all of the ribonucleic acid [RNA] type) which can replicate to the fully infectious state in the tumor cells.
Another type of association between viruses and cells of special significance now for investigations on mammalian tumor cells emerged from the illuminating studies on bacterial viruses which revealed the phenomenon of lysogeny—a phenomenon in which a portion of the viral genetic material, called provirus, is integrated with the genetic material of the host and