Over the past two decades, much effort has been devoted to identifying exemplary drug substrates, or probes, whose metabolism is catalyzed specifically by a given human P450 enzyme. These metabolic probes have been distinguished using expressed recombinant P450s as well as human liver microsomes in the presence of chemical inhibitors and/or specific P450 antibodies. As such, these drugs now serve as positive controls in P450 reaction phenotyping studies performed with new chemical entities. The drug-metabolizing assays described below all meet regulatory agency requirements, and have been chosen due to their high degree of P450 enzyme specificity and ease of use.