The HPTS assay offers considerable flexibility for comparing the transport activity between different anions (X -, as NaX) of interest; chloride can be swapped for 'X -' during the preparation of the internal and external solutions. Care must be taken, however, to avoid misinterpretation of the transport data. A recent study has shown that the misuse of an HPTS selectivity assay that uses NaCl-pH 7.0 IN /NaX-pH 8.0 OUT conditions has led to many false reports of chloride-selective anion transporters. This assay assumes that anion exchange is much faster than the pH gradient dissipation.