“…However, measuring cell penetration of biomolecules is not straightforward. In recent years, chemical biologists have developed a variety of strategies for quantifying passive penetration, total cellular uptake, and cytosolic/nuclear penetration with different detection methods including, but not limited to, fluorescence microscopy, − mass spectrometry, − liquid chromatography–mass spectrometry, fluorescence spectroscopy, − NMR, flow cytometry, − FACS, , qPCR, luminescence, , and ELISA . For cell-based assays, the choice of cell line is critical, and measurements with different methods and/or in different cell lines often produce different results. − In addition, model cell lines such as HeLa are often convenient, but they do not always recapitulate normal cellular physiology.…”