A method for the direct in vivo transformation of cells in and surrounding a solid tumor is disclosed. This method is based on the site-specific delivery of proteins to solid tumors and to tissue surrounding the solid tumor by direct injection of a nucleic acid sequence. In particular, this method is directed to site-specific delivery of nucleic acids encoding major histocompatibility proteins, cytokines, and toxins to a solid tumor. This technique provides for the transfer of vectors and expression of recombinant genes in vivo and allows the introduction of proteins of therapeutic or diagnostic value for the treatment of disease.