A study using family data sets aiming to partition the heritability of gene expression into cis and trans components recently estimated that 37% of the heritability in blood and 24% in adipose tissue are in fact due to cis-regulation 16. Here we confirm these estimates but decompose the cis component further by utilizing IBD estimates in our DZ subjects. We found that between 30-36% of the heritability is due to cis-components but that up to 40% of the heritable cis-effect or 12% of total heritability is missed when only considering common SNPs from cis-eQTL mapping. However, as our analyses were conducted on heritable transcripts in each tissue for which we observed a significant cis-association from the cis-eQTL mapping approach the estimate of the contribution of undetected regulatory effects to cis genetic variance is most likely an underestimate. Although we acknowledge that common SNPs may in some instances tag low frequency variants 25,26 we expect that a subset of the missing cis-heritability still will be accounted for by low frequency and rare variants supporting large-scale exome and genome re-sequencing initiatives for
