Fourth, initial analysis using unrelated participants illustrated the potential for spurious eQTL identification due to expression outliers. Thus, conservatively, we transformed the expression values using the inverse quantile normal transformation, which results in values that precisely fit a normal distribution. These values were used for all primary analyses. Fifth, we evaluated the effects of covariates on gene expression, and found significant associations for plate, hybridization well position, age at blood sampling, sex, time intervals between extraction and hybridization steps, total white and red cell counts, hematocrit, and the top five expression principal components (PCs) (similar to that of surrogate variables). 80 Imputation was performed to estimate a small proportion of missing covariates (2.1%). All heritability and eQTL analyses corrected for these covariates (93 degrees of freedom), and eQTL analyses additionally corrected for the first three genotype PCs.
