In principle, either the peptide or the cargo can be labeled by a reporter group, e.g. a radioisotope [156] or a fluorophore [157] . Fluorescent peptides or cargos have been quantitatively evaluated by FACS [54] or fluorescence correlation microscopy (FCS) [158] or FRET [159] . In all cases, it is crucial to distinguish between internalized and membraneassociated signals. For this purpose, a simple wash step with just buffer is not sufficient to completely remove membranebound peptide/cargo-complexes [54, 55] . Extracellularly bound complexes can be efficiently removed for example by enzymatic digestion with trypsin [160] , acid wash [161] or heparin treatment [55, 162] . Alternatively, discrimination between intra-and extracellular material is possible through chemical modification of extracellular components [163] or fluorescence quenching [164] .