80 peripheral blood mononuclear cell (PBMC) samples collected from living healthy donors (15). The expression data were normalized across all samples for a tissue type (four separate normalizations) per Affymetrix PLIER protocol with a sketch-quantile normalization procedure (Affymetrix Expression Console). Differences in the distribution of mRNA expression levels between SNP genotypes were analyzed using a linear regression model implemented in the program SNPEXPRESS (15) by correcting for age, sex and principle component (PC) scores, separately by tissue. These PCs were calculated from all expression data for both brain and PBMC samples and the first ten PCs were included in the eQTL analysis.
