This is the most characterized fragment of the HIV transactivator protein (TAT). This arginine-rich TAT peptide penetrates plasma membrane directly, but not through endocytosis. This peptide corresponds to the protein transduction domain of the TAT protein and is synthesized with an activated cysteine residue C(Npys), wherein Npys is 3-Nitro-2-pyridinesulfenyl group and is used for activating S of cysteine and for rapid reaction when a thiol group is introduced. The N-terminus is rendered free for applications requiring certain conjugation reactions with a free N-terminal end, and a linker GGG is placed at the C-terminal end, and the peptide has been synthesized with the C(Npys) group. This kind of modification has been used to render this peptide as a cell penetrating and carrier peptide applicable in conjugation studies.