Since cancer is the result of genetic mutations, it should be well suited for correction through gene therapy. Hematological malignancies in which human gene transfer has been performed are leukemias, lymphomas, graft-versus host disease after allogeneic bone marrow transplantation in leukemia, and multiple myeloma. Gene therapy may be used to induce or enhance an antitumor immunological reaction, to correct a genetic defect in the tumor cells, to render the malignant disease more susceptible to conventional therapies, to make the normal host cells more resistant to conventional therapies, or to track cells used for therapy. Gene therapy will probably be most valuable for the eradication of minimal residual disease after the use of conventional therapies. Gene therapy may improve the body’s ability to fight cancer or make the cancer more sensitive to chemotherapy. It may be used to induce or enhance an antitumor immunological reaction, to correct a genetic defect in the tumor cells, to render the malignant disease more susceptible to conventional therapies, to make the normal host cells more resistant to conventional therapies, or to track cells used for therapy. Hematological malignancies in which human gene transfer has been performed are leukemias, lymphomas, risk after allogeneic bone marrow transplantation (BMT) in leukemia and multiple myeloma.