Although IL-15 shares the same signal-transducing receptor subunits (IL-2R<i>β</i> and the common <i>γ</i> chain) with interleukin-2 (IL-2), the two cytokines drive fundamentally different CD8<sup>+</sup> T-cell fates, a distinction that underlies their markedly divergent therapeutic profiles in cancer immunotherapy. The gene discussed is IL15; the disease is cancer.