So far, only a limited number of genome-wide eQTL studies have been performed on various human tissues.11, 12, 13, 14, 15, 16, 17, 18, 19 In most cases, easily accessible peripheral tissues such as human HapMap lymphoblastoid cell lines, lymphocytes or monocytes were investigated. For example, in one of the earliest studies, Morley et al.20 distinguished cis- and trans-effects, depending on the relative location of trait gene and SNP gene to each other. Several later studies found that trans-eQTLs were more difficult to reproduce.12, 13, 14, 15 Only few studies have appeared on internal tissues, including the brain,21 adipose18 and liver.16 The latter study investigated a cohort of 427 human liver samples (in this paper referred to as the ‘Seattle study') and found a multitude of new eQTLs. Furthermore, they showed that the eQTL approach together with network analyses can drive the identification of new susceptibility gene loci for complex disease traits such as type 1 diabetes.16
