method requires the correlation of errors in the estimated SNP effects between two samples (θ), which is estimated by a simple correlation approach at the null SNPs in the cis-region. This approach, however, is not applicable to eQTL or mQTL summary data that have been ascertained by P-value. It will also be challenging to estimate θ if only a small number of cis-SNPs are available in the summary data. We therefore recommend eQTL and mQTL studies to make more cis-SNPs available without ascertainment (e.g., all the cis-SNPs in ±2Mb of a gene or DNAm probe). Despite these caveats, our findings shed light on the genetic architecture underlying the regulation of gene expression across tissues and provide important guidance for studies in the future to identify functional genes for human complex traits.
