Cellular uptake of CPPÀPMO conjugates was investigated using the 3 0 -carboxyfluorescein-tagged PMO (PMOF) and flow cytometry. We chose 2 mM as the treatment concentration because none of the conjugates caused any detectable cytotoxicity at this concentration, as demonstrated by the MTT and PI uptake assays. After treating with the conjugates, cells were treated with trypsin (19) to remove membrane-bound conjugates. We found that a heparin sulfate washing step prior to trypsin treatment did not remove additional membrane-bound conjugates but caused some cellular toxicity (data not shown); therefore, only the trypsin treatment step was used in this study.