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. Cleavage of X allows separation of A from B, unmasking the normal ability of the basic amino acids in B to drag cargo C into cells near the cleavage event. X is cleaved extracellularly, preferably under physiological conditions. D-amino acids are preferred for the A and B portions, to minimize immunogenicity and nonspecific cleavage by background peptidases or proteases.