A central aim of cancer research over the last 20 years has been the identification of genes involved in the development and behaviour of cancer. These genes have formed a foundation for understanding the biological abnormalities within neoplastic cells, have provided information on the function of gene products and shed light on more complex questions, such as the relationships between genes and biochemical pathways. As our understanding of the underlying biology of cancer has grown, the strategies for the development of new therapeutic and preventive agents in cancer have become increasingly dependent upon modulation of critical molecular targets. A number of avenues of investigation have revealed:

• new cancer genes; • the use of biological assays for transforming activity; • primary localization to a small part of the genome (e.g.