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When Steven Rosenberg joined the NCI in 1974, he had a hunch that the immune system could be harnessed to take on cancer. Proving this hypothesis required persistence. For his first major clinical breakthrough — showing that IL-2 could supercharge T cells to treat melanoma — he treated 66 patients with increasingly higher doses of the cytokine, without success, before curing the 67th patient. “That was the really exciting moment,” says Rosenberg. “Prior to that, we didn't know whether an immunotherapy would ever work”.