We are interested in the factors that contribute to inter- and intra-individual differences in the biotransformation of drugs and other xenobiotics. Broadly speaking, this includes endogenous and environmental factors, as well as genetic and non-genetic ones. As most of drug metabolism takes place in the liver, we established a large human liver bank, consisting of 300 clinically well documented surgical samples, and we also use human primary hepatocytes as a cellular model. These biological tools, which we obtain from clinical partners throughout Germany, and our unique selection of modern biomolecular technologies allow us to study a variety of phenomena related to drug therapy and response. In particular we analyze the expression and function of enzymes, their population variability, genotype-phenotype relationships, mechanisms of drug-drug interactions, as well as the role of nuclear receptors, microRNAs and other factors in gene regulation. Following up on discoveries in such basic in vitro studies we develop diagnostic assays to evaluate our results in vivo. To assess the clinical value of our findings, we perform volunteer and clinical studies, either in our clinical study unit or in collaboration with our clinical partners. Our aim is thus to contribute to the understanding of variable drug metabolism and drug response, and to move forward the idea of personalized medicine.