Gene therapy is the insertion of nucleic acids into cells of an organism in order to change or correct its phenotype with the ultimate purpose of treating disease. Whereas traditional drugs act by direct and transient modification of a biological target, the purpose of gene therapy is to reprogram cells. Clinical studies have now demonstrated biological responses or efficacy following gene therapy in patients with genetic diseases affecting the immune system, skin, blood clotting or retina. Positive data have also been reported in patients with AIDS, melanoma and Parkinson disease. Although these studies clearly indicate that gene therapy will be a part of molecular medicine practice in the near future, they also reveal difficulties, limitations and toxicities that require attention.