Identification of cell penetrating conjugates has aided numerous areas of scientific development. The Drosophila transcription factor Antennapedia contains a homeodomain that can be internalised by cells to the cytoplasm and to the nucleus in a receptor-independent mechanism. The key residues for internalisation have been sequenced (RQIKIWFQNRRMKWKK) and used in several studies to aid entry of recombinant proteins into cells. The peptide sequence described, known as penetratin, was fused to a ligand for Grb-2 resulting in inhibition of downstream Grb-2 signalling events.  Penetratin has also been used in vivo to prime cytotoxic T lymphocytes by conjugating short antigenic peptides to the cell penetrating peptide.

On this sequence the N terminal active cysteine has been protected by a 3-Nitro-2-pyridinesulfenyl group. This is a useful modification as it allows this peptide to be used as a carrier peptide in conjugation reactions; this is a particularly useful modification when the peptide-like molecule contains a free thiol group.