Cell-penetrating peptides useful for targeting a therapeutic compound to aselected mammalian tissue, methods for their identification, methods of formingconjugate compounds containing such peptides, and conjugates formed therebyare disclosed. The cell-penetrating peptides are 8 to 30 amino acid residuesin length and consist of subsequences selected from the group consisting of RXR,RX, RB, and RBR; where R is arginine, B is β-alanine, and each X is independently-C(O)-(CHR