US Patent 7989588 Cell permeable peptide

The number of peptides having an ability to bind to a cell or penetrate into a cell is narrowed down by being selectively enriched from a random peptide library with a diversity of not less than one hundred millions of peptides using a phage surface display technique, and then cytoplasmic transfer is evaluated by using protein synthesis inhibition as an indicator by adding to a cell, a fusion body of the selectively enriched peptide and a protein synthesis inhibitory factor (PSIF) that cannot solely penetrate into the cell.