Variables in clinical trial design. Multiple, potentially clinically relevant variables exist between various published clinical trial outcomes treating patients with CD19-targeted, CAR-modified T cells. There are variables in the methodology of CAR gene transfer (1), the design of the CAR (2), the inclusion or exclusion of prior conditioning chemotherapy (3), whether conditioning chemotherapy may reduce tumor burden (4), and whether additional cytokine support with IL-2 is provided exogenously after modified T-cell infusion (5). Whether one or more of these variables are indeed relevant to ultimate clinical outcomes awaits additional multicenter trials resolving these variables by direct comparison to establish the optimal conditions in which these CAR-modified T cells may induce an optimal clinical response.