(A) Correlation between MRP1 mRNA expression by RT-PCR and MRP1 protein expression by flow cytometry. All patients negative by RT-PCR (a very sensitive technique) expressed a fluorescence ratio from 0.81 to 1.4 in the MRP1 protein detection assay. Therefore, the threshold of positivity of 1.4 (horizontal dotted line) was used (with this cut-off of fluorescence ratio, 34% of patients expressed MRP1 protein). (B) Correlation between the effect of probenecid on calcein-AM uptake and MRP1 mRNA expression by RT-PCR in 40 patients. Three patients positive by RT-PCR assay were negative in the MRP1 activity assay. (C) Correlation between the effect of probenecid on calcein-AM uptake and MRP1 protein expression by flow cytometry in 40 patients. One patient positive by flow cytometry assay was negative in the MRP1 activity assay. In some cases, the ratios of fluorescence in MRP1 protein activity assay ranged from 0.72 to 1 (B and C). This was probably caused by variations in cellular uptake of calcein-AM in the experiments. Conversely, the fluorescence ratios of up to 1.28 can also represent some experimental variability (area between the two vertical dotted lines) (B and C). With these thresholds of positivity, there was 7.5% discordance between RT-PCR and functional assays (3 of 40 samples were MRP1+/activity−) and 2.5% between protein detection and functional assays (1 of 40 samples was MRP1+/activity−).