Medicine is undergoing a revolution in the understanding of the mechanisms through which disease processes develop. The advent of genetics and molecular biology to oncology not only is providing surrogate predictors of therapy response and survival which are forming the basis for selection among established treatment options, but is providing targets for new directions in therapy as well. Molecular modification of somatic cells for the purposes of protecting the normal cells from the toxicity of cancer chemotherapy, for the sensitization of the tumor cells to therapy and use of conditionally replicating viral vector have been new directions of cancer treatment which have reached the clinical arena. Advances in molecular pharmacology and vector design summarized in this paper may provide solutions to some of the existing problems in the technology of gene transfer therapy. Continued basic research into the biological basis of human disease, systemic studies of the application of these discoveries to therapy and the improvement of vector for gene delivery all combined may result in advances in this important field of therapy over the next few years.