Because the rates at which therapeutics are cleared from the body can affect their effectiveness, knowing and accounting for the variables that contribute to drug clearance is of utmost importance when designing a drug dosage regimen for patients. The activities of the manifold cytochrome P450 enzymes [(CYPs), the most clinically important of which is often CYP3A] must be considered as they are essential for modification and metabolism of compounds (i.e., therapeutic, xenobiotic, etc.) prior to their excretion. To this end, much research has been expended on trying to identify and develop drug probes that accurately predict the metabolism of CYP3A substrates in individuals. Recently, Benet has written on the futility of such an enterprise; however, other researchers believe the identification of valuable predictive probes is not only possible but crucial.