Recently, a technique to measure cellular uptake of CPPs by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was reported by the group of Burlina [166] [167] [168] . This quantification is based on the addition of an internal standard, i.e. a peptide with a stable isotope label. The method has been used to determine the amount and stability of intact internalized peptides, e.g. Penetratin, R9 and several novel CPPs, and can also be used for the quantification of peptidic cargoes, e.g. an inhibitor of protein kinase C [166] .