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), named penetratin, and used in several studies to aid entry of fusion proteins into cells.

The full 60 amino acid homeodomain was fused to a T cell epitope of the influenza nucleoprotein and successfully internalised into T cells for presentation. The fragment 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 CPP. Penetratin is provided here as a C-terminal amide with a C-terminal 5-carboxyfluorescein (5-FAM), a widely used green fluorescent tag often preferred over FITC due to its high stability; absorbance 492 (nm), 518 emission (nm).