of genetic differences. In these studies, polymorphisms within or near the candidate gene are tested to see if they are statistically associated with relevant phenotypes. Phenotypes might include measures of acute response, patterns of drug use or therapeutic response in patients, or clinical diagnoses of drug dependence or abuse. Other studies use a genome-wide association (GWAS) approach to examine many or most of the common polymorphisms in the genome; these studies do not depend on prior hypotheses about which genes might be important.
