Early attempts to improve the antitumor activity of CAR T cells were based on the clinical use of IL-2 and IL-15 as anti-cancer immunotherapeutics, which as a monotherapy or in combination with adoptive transfer of tumor infiltrating lymphocytes (TILs) (64–66) had significant antitumor activity but were also associated with toxicity at higher doses (66–70). The gene discussed is IL15; the disease is cancer.