Penetratin, the first 16 amino acids of which are derived from the third helix of the Antennapedia protein homeodomain, is known as a protein transduction domain (PTD). Penetratin linked to a phosphodiester oligonucleotide is capable of permeating through neuronal cell membranes and down-regulating genes. It contains a nuclear localization sequence and facilitates internalization of cargo into living cells.