Attempts at cancer gene therapy to date have focused on the treatment of existing neoplasms, using genetic techniques either to target toxic compounds at or enhance the host's response to a tumour. These cytotoxic and immunological approaches have to be tailored to a specific tumour, and often fail because new cellular mutations can overcome the treatment, as for traditional lines of therapy. There has been less attention given to the possibility of using gene therapy to prevent malignant transformation in certain specific contexts for high-risk tissues. Such approaches could add functional proto-oncogenes or tumour suppressor genes, or nullify pathogenic mutations in such genes using antisense or gene replacement techniques. These strategies will not apply to all tumours, but are more likely to be effective where tissue-specific stem cells can be targeted, and for tumours where mutations in one of several genes are involved in carcinogenesis, as for cancer of the colon. Safer vectors for gene delivery will also be required if therapy is to be considered for prevention rather than treatment.