CD4 and neoplasm: By combining high-throughput scRNA-seq, single-cell multiplex cytokine secretion assay, and live-cell imaging of cytotoxic activity, we reveal that CD4+ and CD8+ CAR-T cells are equally effective in direct killing of target tumor cells and that the activation states of these CAR-T cells are highly mixed with Th1, Th2, and Treg responses in the same single cells and largely independent of differentiation status.