The tumor-targeting property of the NGR motif was first reported by Ruoslahti and co-workers in 1998 [16], and then it was revealed that peptides containing the NGR motif bind to an aminopeptidase N (APN or CD13) isoform which is uniquely expressed on the endothelium of tumor vasculature [18,22]. Here, RTN4R is linked to neoplasm.