While the PAMPA method is used routinely to assess passive membrane permeability, it employs an artificial membrane. To obtain a preliminary assessment of the ability of IP+ MPs to enter living cells, we employed the chloroalkane penetration assay (CAPA).{Peraro, 2017 #6544} In this protocol, cells expressing HaloTag protein (HTP) are incubated with a chloroalkane (CA)-tagged molecule of interest (MOI) for a defined period of time. Excess CA-MOI is washed away and the cells are then treated with a CAcoupled fluorescent dye. Finally, excess dye is washed away and the level of fluorescence retained by the cells is assessed by fluorescence cell sorting. This assay is repeated at several different MOI-CA concentrations. The more permeable the MOI-CA conjugate, the less intense the cellular fluorescence will be since the HTP active site is blocked from reacting with the CA-dye conjugate.