Unexpectedly, and in contrast to the safety observed with CAR T/IL15 combination in other disease settings, IL-15-expressing CLL-1 CAR T cells induced a severe and atypical form of cytokine release syndrome (CRS), associated with high levels of circulating tumor necrosis factor-alpha (TNF-α). The gene discussed is IL15; the disease is B-cell chronic lymphocytic leukemia.