Description_x000D_ Biochem/physiol Actions_x000D_ R9-caPep is an octadecapeptide composed of N-terminal membrane-permeant nona-D-arginine sequence linked via a -Cys-Cys- spacer to a C-terminal proliferating cell nuclear antigen (PCNA) interdomain loop-derived sequence (L126-Y133) that mediates PCNA interaction with its binding partners via their PIP-box domains. R9-caPep selectively kills human neuroblastoma cells (IC50 = 10 - 32 μM vs. IC50 = 98 μM/PBMCs and >100 μM/neural crest stem cells), especially those with MYCN gene amplification, by interfering with PCNA-dependent replication fork extension and DSB HR repair, resulting in exacerbated replication stress. R9-caPep intratumoral injection (3 times a week) is reported to suppress SK-N-BE(2)c xenograft tumor expansion in mice in vivo.