Patent ID: 6998106

Claim:
A composition for selectively internally labeling a target cell, comprising a ligand which specifically binds to a surface antigen of a target cell and is internalized by the cell, wherein the ligand is selected from the group consisting of (1) an antibody (2) a fragment of an antibody and (3) a synthetic polypeptide; an oligopeptide which comprises at least one positively-charged amino acid residue and at least one D-amino acid residue, wherein the oligopeptide does not comprise two or more contiguous L-amino acids, wherein the oligopeptide comprises two non-contiguous L-amino acids separated from one another by one or more positively-charged D-amino acids, and wherein the oligopeptide is covalently bound to the ligand; wherein the oligopeptide does not specifically bind to the surface antigen; and a label which is covalently bound to the oligopeptide.