The transforming oncogenes of DNA tumour viruses have proven useful as tools to dissect the mechanisms of complex cellular processes. In particular, studies of the multifunctional proteins encoded by the early region 1A (E1A) of human adenovirus types 2 and 5 have provided insight into the regulation of cellular gene expression, growth and differentiation. Despite their well known ability to immortalize primary rodent cells and transform them in cooperation with a second oncogene, the E1A proteins also exhibit significant anti-tumour/tumour suppressive activity. This review focuses on the surprising ability of E1A to function as a tumour suppressor gene.