Cell-penetrating peptides useful for targeting a therapeutic compound to a selected mammalian tissue, methods for their identification, methods of forming conjugate compounds containing such peptides, and conjugates formed thereby are disclosed. The cell-penetrating peptides are 8 to 30 amino acid residues in length and consist of subsequences selected from the group consisting of RXR, RX, RB, and RBR； where R is arginine, B is ²-alanine, and each X is independently -C(O)-(CHR 1 ) n -NH-, where n is 4-6 and each R 1 is independently H or methyl, such that at most two R 1 's are methyl. In one embodiment, X is a 6-aminohexanoic acid residue.
