Alcoholism is a quantitative disorder that is caused by the combined influences of numerous genes (i.e., quantitative trait loci [QTLs]) and environmental factors. To identify QTLs for alcoholism, researchers compare subject groups (e.g., inbred mouse strains) that differ in both their genetic makeup (i.e., genotype) and alcohol-related trait (e.g., sensitivity to certain alcohol effects). Using statistical tests one can then determine whether a specific gene or DNA region contributes to the trait of interest. This strategy requires that the relevant gene exists in several variants (i.e., is polymorphic). To conduct such QTL analyses, researchers study either a large population of mice that all differ in their genotypes or compare several strains, each of which has a fixed genotype. However, QTL analyses still have several limitations. Nevertheless, such studies already have identified several DNA regions and genes that may affect the response to alcohol and thus may contribute to the risk for alcoholism.