We conclude with several caveats. First, our estimation of rb is based on genes expressed in both brain and blood (i.e., genes only expressed in one tissue were not included in the estimation). Therefore, the estimate of rb needs to be interpreted with a restriction to genes expressed in both tissues. Although only a quarter (4257) of all genes were selected in our analysis (with at least one cis-eQTL at PeQTL < 5 × 10−8 in GTEx-muscle), up to 90% of those selected genes were expressed in both brain and blood, reflecting the high proportion of all genes expressed in both tissues. Second, we focused our analyses only on cis-eQTLs and cis-mQTLs because trans-eQTLs and trans-mQTLs data were not available in most data sets used in our study. Although most SNP-based heritability for gene expression levels are attributed to cis-eQTLs9, trans-eQTLs may also have an important role in regulating gene expression especially for tissue-specific effects14. The methods developed in this study can be applied to trans-eQTL/mQTL data with minimal modification. Because the variance explained by individual trans-eQTL/mQTL is small on
