addition, the inclusion of the p-2 microglobulin gene, with which 
class I MHC genes normally associate, allows synthesis of the 
complete histocompatibility molecule, which is composed of these 
two chains. Ordinarily, these two gene products are co- 
transported to the cell surface, and some human melanoma cells do 
not express endogenous p-2 microglobulin, thus limiting their 
ability to stably express class I on the cell surface. We have 
found that the inclusion of the p-2 microglobulin gene on the 
same plasmid allows for the expression in these otherwise 
resistant cells and improve expression in other cells, thus 
overcoming a potential mechanism of resistance. A potential 
future modification of the vector involves the expression of a 
cytokine gene in addition to class I MHC and p-2 microglobulin. 
The elaboration of cytokines such as IL-2 or GM-CSF could further 
stimulate T cells immunity against the tumor locally and improved 
recognition of tumor-associated antigens. In experimental animal 
models, the introduction of IL-2 has allowed for improved anti- 
tumor efficacy ((6); see Section 13, Preliminary Data). To 
maximize the safety concerns for vectors which produce cytokines 
in the future, we would include the cytokine gene on the same 
transcript as the class I MHC gene, linking the expression of the 
cytokine gene to expression of the foreign histocompatibility 
antigen. In this way, cells which contain the cytokine gene 
would be eliminated after several weeks in vivo, minimizing any 
concerns regarding persistent expression of cytokines in vivo. 
At present, such a vector is not yet available; however, we would 
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