In general, the HPTS assay employs synthetic unilamellar vesicles (200 nm) composed of POPC or EYPC containing HPTS (1 mM) and are suspended in and contain a solution of NaCl (100 mM) buffered to pH 7.0 using 4-(2-hydroxyethyl)-1piperazineethanesulfonic acid (HEPES, 10 mM). A fluorimeter is used to monitor changes in HPTS fluorescence emission due to dissipation of the pH gradient by transporter-facilitated Cl -/H + symport or Cl -/OH -antiport (Figure ). Transporter activity can then be assessed according to the subsequent pH dissipation across the lipid bilayer. However, it should be stated that due to the intrinsic flexibility of this assay, conditions like the type of phospholipid, liposome size, pH, buffer systems, salts, and solvent media, can be changed to suit the needs of systems being tested. By performing Hill analysis on the data obtained from the HPTS assay, the activity (EC50) of either H + /Cl -symport or OH -/Cl -antiport can be determined.