Microsomal enzymes are the drug-metabolizing enzymes that remain associated with fragments of the smooth endoplasmic reticulum after liver tissue is homogenized and subjected to differential centrifugation, yielding a membrane-bound vesicle fraction termed microsomes ^[1]. The microsomal drug-metabolizing system of the liver consists of two protein components—cytochrome P450 and NADPH-cytochrome c reductase—together with the lipid phosphatidylcholine; cytochrome P450 serves as the binding site for oxygen and substrate, the reductase shuttles electrons from NADPH to cytochrome P450, and the phospholipid facilitates electron transfer between them ^[2]. This system is classified as a mixed-function oxidase or monooxygenase: it requires both NADPH and molecular oxygen, and it incorporates one atom of oxygen into the substrate while reducing the second oxygen atom to water ^[3]. Microsomal enzymes occupy the principal position in phase I xenobiotic biotransformation, catalyzing aromatic and aliphatic hydroxylation, N-, O-, and S-dealkylation, deamination, dehalogenation, sulfoxidation, N-oxidation, and azo- and nitro-reduction ^[2][3]. Within the taxonomy of pharmacogenetics and drug metabolism, the term designates this membrane-associated, cytochrome P450–centered enzyme system as the object of studies on substrate specificity, enzyme induction, competitive and mechanism-based inhibition, and interindividual variability in hepatic drug clearance [3]. The microsomal fraction also retains phase II enzymes situated in the endoplasmic reticulum, notably UDP-glucuronosyltransferases, whose activity is influenced by the membrane environment in which the microsomal preparation embeds them ^[4].