Hydrolyzed calcein AM (e.g., calcein) is a hydrophilic compound that can be retained in live cells with intact plasma membranes for several hours to days depending upon cell type, cell morphology, or experimental conditions. In a cell-based assay performed by Miles et al., 2016, prostate cancer cells loaded with calcein AM were imaged at 0.5, 2, 6, 12, 24, and 36-hour marks. At the 36 hour mark, cells are still moderately fluorescent; however, there was an increase in background fluorescence. One reason for noise increase is the extrusion of calcein by organic anion transporters (OATs), which play a key role in regulating anion balance. Probenecid, which is an OAT inhibitor, can be added to the calcein AM working solution to improve the intracellular retention of de-esterified calcein and reduce interference.