The factors for the individual differences include both genetic and environmental factors. Genome-wide association study (GWAS) was first developed in RIKEN in 2002 and has now become a major research technology. In GWAS, the associations between genomic variations and the variations in the traits are analyzed by statistical genetics. As many as 100,000-2,500,000 polymorphic loci are distributed over the whole genome, and the associations between either genotypes or alleles and the phenotypes are analyzed. The phenotypes are either diseases, physical data or clinical laboratory data, and it was found that the targets of the available drugs are often included in the genes that were found to be associated with the diseases. These data indicate that the results from GWAS may be applied to the development of new drugs (GWDD; genome-wide drug discovery). GWAS started to influence pharmacogenomics in which the associations between genomic variations and drug reactions are studied. Genes associated with statin-induced severe muscle damage, efficacy of interferon for C-type hepatitis, ribavirin-induced hemolytic anemia and carbamazepine-induced severe skin eruption were found by GWAS. In 2010, "Guidelines for clinical studies and tests in which pharmacogenomics is applied" was released as a draft.