Transporters are membrane-bound proteins that control the access of endogenous and xenobiotics (drugs) to various sites in the human body. They influence drug pharmacokinetics and pharmacodynamics (both benefit and risk) by affecting a drug's absorption, distribution, metabolism (via control of access to metabolizing enzymes), and excretion (ADME) and by controlling drug concentrations at the site of action. Like metabolizing enzymes, transporters have binding sites that are saturable and can be inhibited or induced.