RTN4R and neoplasm: APN/CD13 has been identified as an NGR receptor; meanwhile, the use of radiolabeled NGR peptides, as well as cytotoxic or antiangiogenic substances such as doxorubicin, 5′-fluorouracil, coagulase, or tissue factor coupled with NGR motifs represents a widely applied principle in CD13-targeted experimental tumor therapy [15,16,17,111,112] (which has already been applied to humans [11]), of tumor imaging [14].