ANPEP and neoplasm: Small peptides containing the NGR motif (asparagine-glycine-arginine) bind to CD13 (aminopeptidase N) which is overexpressed on endothelial cells of proliferating blood vessels in both mice and humans, allowing NGR-coupled tTF to engage endothelial cells on the luminal side of tumor-associated vasculature with some level of tumor selectivity.