CD63 is a tetraspanin enriched on the surface of exosomes and regarded as an exosomal marker (14). We selected for peptides that specifically bind to the second extracellular loop of CD63 and char- acterized three candidate peptides in CD63-expressing cells and exosomes. One peptide in particular, CP05, enabled direct painting of exosomes with different moieties irrespective of the origin of exosomes