The in vitro study of xenobiotic metabolism in the pharmaceutical industry has routinely been used to help evaluate properties of new molecular entities and help optimize these properties to allow for evaluation of the pharmacologic target in animal models, along with safe dosing in humans. Subcellular fractions, abundant, commercially available, and often well-characterized before purchase, are the most often used in vitro tools to predict both in vivo clearance and the potential for drug interactions via enzyme inhibition. In vitro studies not only allow for evaluation to predict drug properties in human, but also decrease the use of animals in drug discovery and development.