To investigate sharing of the top eQTLs among tissues, we assessed sharing of effects by sign and by magnitude (effects have the same sign and are within a factor of 2 in size). Because of the large differences between brain and non-brain tissues, we also show results separately for these subsets. The results (Table 1, Fig. 5) confirm extensive eQTL sharing among tissues, particularly among brain tissues. Sharing by sign always exceeds 85%, and is as high as 96% among brain tissues. (Furthermore, these numbers may underestimate the sharing by sign of actual causal effects due to the impact of multiple eQTLs in LD; see Supplementary Note and Supplementary Fig. 4.) Sharing by magnitude is necessarily lower because sharing by magnitude implies sharing by sign. On average, 36% of tissues show an effect within a factor of 2 of the strongest effect at each top eQTL. However, within brain tissues this increases to 76%. Thus, not only do eQTLs tend to be shared among brain tissues, but effect sizes tend to be homogeneous. Because our analyses were based on the top eQTLs, our results reflect patterns of sharing only among stronger cis eQTLs; weaker eQTLs may show different patterns.
