We now understand neoplastic transformation to be the consequence of multiple acquired genetic alterations. These changes in aggregate confer the various phenotypes that constitute the clinical features of cancer. Although only rare human cancers derive from a viral etiology, the study of DNA tumor viruses that transform rodent and human cells has led to a greater understanding of the molecular events that program the malignant state. In particular, investigation of the viral oncoproteins specified by the Simian Virus 40 Early Region (SV40 ER) has revealed critical host cell pathways, whose perturbation play an essential role in the experimental transformation of mammalian cells. Our recent work has re-investigated the roles of two SV40 ER oncoproteins, the large T antigen (T) and the small t antigen (t), in human cell transformation. Co-expression of these two oncoproteins, together with the telomerase catalytic subunit, hTERT, and an oncogenic version of the H-RAS oncoprotein, suffices to transform human cells. T inactivates two key tumor suppressor pathways by binding to the retinoblastoma protein (pRB) and p53. The ability of t to transform human cells requires interactions with specific subunits of the heterotrimeric protein phosphatase 2A (PP2A), an abundant family of serine-threonine phosphatases. In this application, we propose to use molecular biological, genetic, and biochemical approaches to investigate the molecular interactions that lead to human cell transformation by t. Investigating the consequences of the interaction of t with PP2A will identify new pathways critical for cancer development as well as novel targets for cancer therapeutics.