This article is available online at http://www.jlr.org Phenotyping of cytochrome CYP3A seems to be of special importance because enzymes belonging to this family, particularly CYP3A4, are involved in the metabolism of more than 50% of currently prescribed drugs. However, the selection of an appropriate CYP3A phenotyping substrate and metric is still a matter of discussion, although some CYP3A test substrates, including midazolam, erythromycin, alprazolam, and nifedipine have been proposed ( 15 ). However, administration of these drugs poses challenges in specifi c patient populations such as pediatric and elderly patients, transplant recipients, and cancer patients, in whom administration of probe drugs may have a negative impact on patient safety. Identifi cation of an appropriate endogenous CYP3A substrate and corresponding metrics would be a promising alternative, allowing simple and possibly broader application of the phenotyping approach. Oxidation of the endogenous compound cortisol to 6  -hydroxycortisol has been shown to be a CYP3A-dependent pathway in humans. Consequently, a urinary 6  -hydroxycortisol-to-cortisol ratio has been proposed as a suitable phenotyping marker for the assessment of CYP3A activity ( 16,17 ). However, the sensitivity of the method to detect small changes in enzyme activity is low because of pronounced background variation ( 17 ). Another compound, 4  -hydroxycholesterol, which is identifi ed as one of the major oxysterols in humans, was reported to be formed solely by CYP3A4 ( 18 ). Markedly elevated concentrations of 4  -hydroxycholesterol were found in patients treated with CYP3A4 inducers ( 19-23 ), and decreased concentrations in patients treated with CYP3A4 inhibitors ( 22-24 ). Furthermore, a relationship between blood 4  -hydroxycholesterol concentration and the number of active CYP3A5*1 alleles has been demonstrated, suggesting that 4  -hydroxycholesterol is formed not only by CYP3A4 but also by CYP3A5 ( 25 ). Accordingly, 4  -hydro xycholesterol has been proposed to be a potential endogenous biomarker of CYP3A activity. Impaired renal function alters the clearance of many drugs mainly by decreasing their renal elimination. However, several studies in rats ( 1-6 ) and in patients ( 7-9 ) have shown that renal failure also decreases the metabolic clearance of drugs, particularly those metabolized by cytochrome P450 (CYP). The underlying causes of altered CYP functional expression observed in kidney disease remain unclear, but several studies indicate that uremic toxins ( 10-13 ) and infl ammation ( 14 ) may play a role via transcriptional or translational modifi cations of CYP enzymes.