Particular eukaryotic genes which play integral roles in the control of normal growth and differentiation programs are targets for mutagenic events which lead to the generation of malignancies. These genes, called proto-oncogenes can be activated to the oncogenic state by amplification, point mutation, deletion or chromosomal translocation. The protein products encoded by oncogenes include protein kinases, G-proteins, growth factors and nuclear transcription factors. It is likely that oncogene activation by viral infection or in concert with virally-induced immunodeficiencies is responsible for the aetiology of human tumours.