CD19 and neoplasm: Using the CRISPR/Cas9 system, Su et al. performed PD-1 gene knock-out in patient-derived T cells and observed enhanced cytokine production and cytotoxicity in vitro.81 Rupp et al. further applied CRISPR/Cas9-mediated PD-1 knock-out in anti-CD19 CAR T cells and demonstrated improved clearance of PD-L1+ tumor xenografts in vivo.82 Hu et al. observed similar results with PD-1 knock-out in anti-mesothelin CAR T cells.83