CYP-3A4 (0.5 nmol/ml) was incubated with 50 μM mifepristone in a reconstituted system at 37°C for 15 min as described in Methods and Materials. Aliquots of 50 μl of incubation mixture were diluted into 0.95 ml of 50 mM HEPES buffer (pH 7.5) for determinations of testosterone 6β-hydroxylation activity.