PEP-1 peptide, which has 21 amino acid residues, is a peptide carrier for the noncovalent delivery of proteins into cells. it consists of three domains: (a) a hydrophobic tryptophan-rich motif containing five tryptophan residues (KETW WETWWTEW); (b) a hydrophilic lysine-rich domain (KKKRKV) derived from the nuclear localization sequence (NLS) of simian virus 40 (SV-40) large T antigen; (c) a spacer domain (SQP), separating the two domains mentioned above, containing a proline residue, which improves the flexibility and the integrity of both the hydrophobic and the hydrophilic domains.