Pep-1 is one of the synthetic cell-penetrating peptides (CPPs), which has been successfully used to deliver a variety of proteins and other biopharmaceutical macromolecules into cells in a non-disruptive way. It is a CPP with primary amphipathicity that comprises a tryptophan-rich so-called `hydrophobic' domain, a hydrophilic domain derived from an NLS (nuclear localization signal) of SV40 (simian virus 40) large T-antigen, and a spacer between them. The cysteamine group in C terminal seems to play a crucial role in the delivery efficiency of cargoes into cells.