Recently, using affinity selection (biopanning) of phagedisplayed peptide libraries, researchers have discovered molecules that are expressed on tumor blood vessels exclusively [16, [34] [35] [36] . The strategy for identifying tumortargeting ligands and developing ligand-mediated targeted therapy is shown in Figure 1 . Researchers have used in vivo affinity selection of phage libraries to identify peptides that interact with the molecules found on endothelia in tumors [34, 36] . The NGR peptide motif targets angiogenic blood vessels [36] and the tumor-homing property of NGR motif relies on recognition of a CD13 isoform selectively expressed within tumor blood vessels [54] . Compared with the nontargeting liposomal doxorubicin (Caelyx), NGR peptide-conjugated Caelyx significant improvements in survival was seen in clinically relevant animal models of neuroblastoma, ovarian, and lung cancers [17] . Another peptide, SP5-52, has been found to recognize blood vessels created in tumors but not normal blood vessels in severe combined immunodeficiency (SCID) mice bearing solid tumors. Several selected phage clones display Pro-Ser-Pro, a motif crucial to peptide binding to tumor neovasculature [13] . Several tumor homing peptides have been found to bind to blood vessels in surgical specimens of human cancer and they have also been found to home to tumor tissues of different human tumor xenografts as confirmed by in vivo homing assays [16] . These studies found a greater correlation between increased tumoral accumulation of the targeting liposomes and antitumor efficacy than the accumulation of free drugs or drugs formulated in the nontargeting liposomes [2, 13, 16 ].