CR2 and central nervous system cancer: Our result shows that HB1.F3.CD21 NSCs are non-tumorigenic in the peritoneal setting (Fig. 4c), consistent with our current clinical data in the glioma setting, as patients in phase I trials of allogeneic NSC-mediated enzyme/prodrug and CRAd-S-pk7 treatments have tolerated multiple intracranial administrations without adverse events or evidence of secondary tumorigenicity [16].