A generic structure for the peptides of the present invention includes A-X-B-C, where C is a cargo moiety, the B portion includes basic amino acids, X is a cleavable linker sequence, and the A portion includes acidic amino acids. The intact structure is not significantly taken up by cells; however, upon extracellular cleavage of X, the B-C portion is taken up, delivering the cargo to targeted cells. Cargo may be, for example, a contrast agent for diagnostic imaging, a chemotherapeutic drug, or a radiation-sensitizer for therapy. X may be cleaved extracellularly or intracellularly. The molecules of the present invention may be linear, cyclic, branched, or have a mixed structure.
STATEMENT OF FEDERALLY-SPONSORED RESEARCH
 This work was supported in part by grants from the Department of Energy, DE-FG03-01ER63276 and from the National Institutes of Health Grants DK54441, GM54038 and NS27177 (NINCDS). The government may have certain rights in this invention.