The DNA sequence that encodes the protein cytokine interleukin-2 (IL-2). When introduced as the complementary DNA (cDNA) form into tumor cells by, for example, a genetically engineered adenovirus vector, the transfected IL-2 cDNA expresses IL-2 which may activate antitumoral natural killer cells and elicit an antitumoral cytotoxic T-cell response, resulting in an inhibition of tumor progression.