TAT (YGRKKRRQRRR) represents amino acids 47-57 of the human immunodeficiency virus type I (HIV-1) TAT protein. The positively charged protein transduction domain of the HIV-1 TAT protein (TAT-PTD; residues 47-57 of TAT) rapidly translocates across the plasma membrane of living cells. TAT (YGRKKRRQRRR; TAT-PTD) can cross the hydrophobic lipid bilayer of a cell membrane simply by passive diffusion.