The bovine papillomavirus type 1 (BPV-1) has served as a model for unravelling the molecular genetics of the papillomaviruses. BPV-1 transformation of rodent cells in tissue culture has provided a means to study the viral functions involved in latent infection of cells and in the induction of cellular proliferation functions. BPV-1 has been shown to encode two independent transforming genes, each of which can induce cellular transformation in susceptible rodent cells. These two genes apparently act synergistically in transforming mouse C127 cells. Deletion mutagenesis studies have shown that the expression of one of these genes (E6) is required for efficient tumorigenesis and anchorage independence. BPV-1 also encodes functions which may act indirectly to affect transformation. BPV-1 contains transcriptional enhancers which can act in a position-independent and orientation-independent manner to increase the transcriptional activity of a heterologous gene. One of these elements, which is located in a non-coding region of the genome, can be trans-activated by a specific viral gene product encoded by the E2 open reading frame. Mutations which eliminate this trans-activation function also have a dramatic effect on transformation and on stable plasmid maintenance.