Our in vivo pharmacokinetic studies in mice showed that liposomal doxorubicin dramatically changed the transportation and distribution of doxorubicin in the heart, lungs, kidneys, and liver in mice (Figs. 4 and S5) . These results echo other similar investigations [43, 44] . With prolonged presence of liposomes in circulation, more doxorubicin was taken up by tumor cells than conventional doxorubicin administration (Fig. 4) ; this finding was further confirmed in fluorescence signaling of doxorubicin in tumor tissues (Fig. 5A) . This passive targeting phenomenon of non-ligand conjugated liposomes is called the ''enhanced permeability and retention effect'' [45, 46] . In our study, the doxorubicin concentration in the LD-treated group was three times that of the (Table S2) . However, the tumors in the SP5-2-LD group had even higher doxorubicin concentration that was 1.9-and 2.1fold higher than those in the LD and MP5-2-LD groups (Table  S2 ). The bioavailable drug in the nuclei of cancer cells in the SP5-2-LD was also 2.6-and 2.4-fold higher than those in the LD and MP5-2-LD groups (Table S2 ). These results indicated that this peptide directly delivered the chemotherapeutic drug to intended targets. Enhancement of drug accumulation in tumor tissues correlated with the increased therapeutic efficacy (Figs. 3, 4 and 5) . The peptide-functionalized liposomes were found to have important clinical potential in a targeted drug delivery system. In order to for them to be used clinically, however, a final targeting liposome construct will need to be selected after each component, i.e. peptide ligands, conjugation methodologies, and liposomal drugs, have been optimized.