A drawback of the present study lies in the use of whole blood instead of neuronal tissue for the measurement of mRNA expression levels. As neuronal tissue is inaccessible in living ALS patients, one could consider the use of human neuronal tissue from autopsy. However, in post-mortem material of ALS patients, most affected motor neurons will have degenerated and one would be investigating exclusively end-stage disease expression profiles. We have investigated the proportion of overlapping eQTLs between our study and other studies, including two studies on human brain tissue (Table S4) [24], [26], [45], [46]. Studies of the genetics of gene expression appear to have modest overlap in the eQTLs identified. For example, 36.1% of genes mapped by a cis eQTL in lymphocytes were identified in a study using lymphoblastoid cell lines [24], [45]. A smaller overlap (22%) was found between two studies on brain tissue, which may partly be due to low statistical power [26], [46]. In the present study, 37 – 52% of the genes mapped by cis eQTLs in human brain tissue studies appeared to be present
