"Gene therapy approaches fall into two main categories: (1) the transfer of a gene that stimulates the immune system and (2) the insertion of genes within the tumor that sensitizes the tumor to a relatively nontoxic 'prodrug.' Both these approaches show a great deal of promise."
The second part of Mixson's answer echoes the research described by Jacobson:
"Therapies that inhibit tumors' blood vessel development also seem to have considerable merit. Blood vessel development is technically known as angiogenesis, so the drugs that inhibit this process are called antiangiogenic. These antiangiogenic drugs include antibiotic derivatives, peptides (small protein fragments) and peptide derivatives. Many of the peptides being tested have been isolated from proteins that are secreted by tumors. Blood vessel development is critical for the growth of any solid tumor, such as breast, colon or lung cancer. Thus, any therapy that can inhibit the tumors' blood supply should be effective against multiple types of tumors. This area of research is receiving a good deal of attention, and there are already clinical trials in this field.