PEP-1 peptide, which has 21 amino acid residues, is a known carrier peptide that delivers full-length native proteins in vitro and in vivo. Pep-1 peptide, consists of three domains: (1) a hydrophobic tryptophan-rich motif containing five tryptophan residues (KETW WETWWTEW); (2) a hydrophilic lysine-rich domain (KKKRKV) derived from the nuclear localization sequence (NLS) of simian virus 40 (SV-40) large T antigen; and (3) 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. In standard cell culture conditions, Pep-1 localizes rapidly, in <10 min, to the nucleus of human HS-68, murine NIH-3T3 fibroblasts, or Cos cells. Similar experiments, performed by incubating cells for 30 min at 4°C before transfection, yielded essentially the same result, indicating that Pep-1 internalization is independent of normal endocytosis.