Pharmacogenomics is the study of the stratification of the pharmacological response to a drug by a population based on the genetic variation of that population. It has long been known that different individuals in a population respond to the same drug differently, and that these variations are be due to variations in the molecular receptors being affected by the drug, or to differences in metabolic enzymes that clear the drug. Pharmacogenomics is the science of studying these variations at the molecular level. By identifying and classifying all the tolerable variations of a molecular receptor known to exist in a population, and then performing systematic studies of the effect of the drug on each of the variants, one can hope to predict or constrain the use of the drug to different subgroups. Applications of pharmacogenomics include reducing side effects; customized drugs; improved clinical trials; and the rescue of some drugs that have been banned due to severe side effects in a small percentage of the eligible population.