The main purpose of probenecid in calcium flux flow assays is to enhance the retention of these dyes and achieve optimal signal sensitivity. Probenecid is a compound which inhibits organic anion transporters and prevents the expulsion of fluorescent dyes from the cells. In a standard calcium flux assay for CHO cells using Fluo-3, Fluo-4, or similar commercial kits, a concentration of 2.5 mM probenecid is recommended.