To confirm the specificity and selectivity of eFluxx-ID® probes further, we employed cultured cell lines that over-express a single transporter and are reported to exhibit multidrug resistance, HCT-15 (colorectal adenocarcinoma, overexpression of MDR1 [22]), HepG2 (hepatocellular carcinoma, overexpression of MRP [23], [24]), and HL-60/MX1 cell line (acute promielocytic leukemia, where mitoxantrone resistance is attributed to BCRP overexpression [33], [34]). The gene discussed is ABCG2; the disease is hepatocellular carcinoma.