ABCG2 and chronic kidney disease: Since reported plasma concentrations of indoxyl sulfate (Table S1), including results shown in Fig. 2b, did not increase to the millimolar order in mice and human CKD conditions, the Km value indicated that ABCG2-mediated indoxyl sulfate transport is not saturated under (patho)physiological conditions and that ABCG2 can maintain its transport activity even under high concentrations of indoxyl sulfate.