One example of Pgp and MRP1 activity quantified by the effect of probenecid ± CsA (modulators of MRP1 and Pgp, respectively) on the level of calcein-AM uptake. Cell fluorescence (A) without modulator, (B) with probenecid, (C) with CsA, and (D) with both probenecid and CsA together. The results were calculated as the ratio of drug fluorescence with modulator divided by drug fluorescence without modulator after subtraction of the fluorescence of the control. For this example the ratios were 1.51 with probenecid (which quantified MRP1 activity); 1.7 with CsA (which quantified Pgp activity); and 2.66 with both probenecid and CsA (which quantified the combined activity of MRP1 and Pgp).