CYP3A5 is a cytochrome P450 monooxygenase that catalyzes the oxidative metabolism of endogenous substrates and xenobiotics. It functions by inserting one oxygen atom into substrates while reducing the second to water, utilizing electrons from NADPH via cytochrome P450 reductase 1234. CYP3A5 exhibits high catalytic activity for steroid hormone metabolism, catalyzing 6beta-hydroxylation of testosterone, progesterone, and androstenedione, as well as catechol estrogen formation from estradiol and estrone 34. The enzyme plays a critical role in retinoic acid metabolism, catalyzing the oxidative conversion of all-trans-retinol to all-trans-retinal and further metabolizing retinoic acid to 4-hydroxyretinoate 12. CYP3A5 also metabolizes approximately 30-50% of known drugs, including calcium channel blockers like nifedipine and amlodipine, and immunosuppressive agents like cyclosporine and tacrolimus 546. CYP3A5 is predominantly expressed in lung tissue and exhibits substantial inter-individual variability due to genetic polymorphisms 7. CYP3A5 polymorphisms significantly influence drug metabolism and clinical outcomes; for example, tacrolimus trough levels and acute rejection rates in transplant patients vary dramatically with CYP3A5 genotypes 6, and CYP3A5 polymorphisms associate with myocardial infarction risk and hypertension development 89. Personalized dosing based on CYP3A5 genotyping is clinically important for optimizing therapeutic efficacy and minimizing drug toxicity.