This is the first column of a series discussing how advances in pharmacogenomic information (PGI) and molecular biology are leading to changes in the product labels of existing drugs and providing new targets for drug discovery. This column first introduces the concept of PGI and defines related terminology. The authors then discuss how new information on genetic variations in patient responses to drugs has led to revisions in the product labels of many already marketed drugs. Rapidly expanding PGI has also led to the development of new drugs with novel mechanisms of action. Such drug development has been especially common in oncology, with new agents being developed to target genetically specific forms of cancer. The authors review how genetically determined variations in the pharmacokinetics and pharmacodynamics of a drug in a specific patient can make that patient "sensitive" or "resistant" to the effects of that particular drug. This type of PGI is expanding the concept of "special populations" to include patients with genetically determined differences in pharmacokinetics and/or pharmacodynamics. The second column in this series will explain how increased knowledge of molecular pharmacology and PGI has resulted in the revision of product labels for drugs already on the market, using pimozide as an example. The third column in this series will deal with the discovery of new drugs with novel mechanisms of action, with a focus on oncology drugs. The last column in the series will discuss the need to make this knowledge readily accessible to clinicians at the time and point of therapeutic care.