The genetic changes found in human neoplasms suggest that hematopoietic tumors develop mainly from inappropriate expression (usually overexpression) of a growth promoting gene (oncogene). In contrast, the progression to malignancy of carcinomas occurs mainly through loss of tumor suppressor genes. Gene therapy might be used to turn off an activated oncogene, eg, by antisense treatment, whereas gene therapy to overcome tumor suppressor gene loss necessarily would focus on gene replacement in the tumor cell or pharmacologically substituting for lost gene function. On the other hand, the protein products of mutations that activate oncogenes or that inactivate tumor suppressor genes are both potential tumor antigens. Increasingly, characterization of the molecular changes that contribute to the malignant phenotype provides information impacting on tumor diagnosis and patient prognosis.