differences in gene expression level among cell types rather than cell-type-specific genetic effects. On the other hand, however, the strong between-tissue correlation in cis-eQTL effects does not contradict the result that many genes showed differential expression between brain and blood because the difference in cis-eQTL effect is almost independent of the mean difference in gene expression level (Fig. 3). Our results reinforce that very large sample sizes are needed to generate eQTL data sets in a specific tissue (e.g., blood) to increase the power of detecting cis-eQTLs regardless of the relative expression level of the tissue.
