Because no single assay provides a complete analysis of the transformed phenotype, transactivation assays complement the cell growth and tumorigenicity analyses of oncogene function. Transactivation of ORE-containing genes is such a common feature of a diverse variety of viral and cellular oncogenes that it can be considered one aspect of the oncogene-induced phenotype. After the initial identification of oncogenes that activate transcription, studies of the mechanisms of activation and the identification of the downstream target genes should lead to a better understanding of the events leading to cellular transformation. The fact that cell type specificity of transactivation and transformation can be similar means that the transactivation assay may be a useful tool in dissecting cell type-specific transformation. The transactivation assay of oncogene function also has the advantage that it is easy to perform and significantly more rapid than assays based on altered cell growth. This is of particular advantage when one wishes to examine the function of a large number of oncogene mutants generated in vitro. Overall, transactivation assays provide another tool for examining transforming potential and a starting point for the analysis of the downstream targets of oncogenes.