A relatively new approach to identify unknown functional genetic variants that modulate gene expression, also termed ‘genetical genomics,' is the mapping of expression quantitative trait loci (eQTLs) using genome-wide association (GWA) methods in cohorts of unrelated individuals.2, 7 In this strategy, individual transcript levels are determined in a selected tissue or cell type using microarrays. In genomic DNA of the same individuals, in the order of 105 to 106 SNPs are genotyped in parallel. By considering each individual gene transcript as a quantitative trait, association analysis identifies SNPs that are significantly associated with expression.8, 9 Thus, the eQTL strategy differs from the typical GWA studies, as the majority of the >1000 published GWA studies typically focused on a single or only a few complex phenotypes.10
