This type of intervention involves the correction or attempted correction of genetic defects in any of the cells of the body, with the exception of the germ or reproductive cells. One such example would be the insertion of cytokine genes, such as interleukin-2, tumor necrosis factor, or granulocyte-macrophage colony-stimulating factor, into a patient's malignant cells to produce an immune response (the production of cytotoxic T cells that are specifically targeted to the tumor). .
Type 2 genetic intervention involves the correction or prevention of genetic deficiencies through the transfer of properly functioning genes into reproductive cells. This might be accomplished by DNA transfer into one of the pronuclei of the zygote, the delivery of DNA into a four or eight-cell embryo by a vector, or the use of embryonic stem cells. However, it is necessary to replace the faulty gene rather than add a gene (the usual technique in current somatic cell gene therapy). In altering the germline, gene addition would be unsatisfactory because it is not possible to predict the effects of a mixture of the normal gene and the mutated gene with respect to regulatory signals necessary for normal growth and development. Therefore, reliable, predictable gene replacement is a needed advance before germ-line intervention can be seriously considered. .
Type 3 and type 4 genetic interventions would involve the use of somatic cell or germ-line gene modifications, respectively, to affect selected physical and mental characteristics. These procedures would attempt to influence such features as physical appearance or mental abilities. A principal difference in these uses of genetic modification is that they could be directed toward healthy people who have no evidence of genetic deficiency diseases. Further, type 4 genetic intervention, if successful, could assure that the enhancement would be passed on to succeeding generations.
