DNA was extracted from blood samples in all participating studies. A detailed overview of SNP genotyping, including the platforms used and subsequent quality control, is given in Table 2. The studies used Illumina platforms, except for SardiNIA and NTR/NESDA, which used Affymetrix and Perlegen platforms, respectively. Genotype data were checked in each study independently, using slightly different inclusion criteria. Among the basic checks that were performed were checks for European ancestry, Mendelian errors, gender inconsistencies and high genome-wide homozygosity. Checks for relatedness were carried out in those samples that aimed to use unrelated individuals. Genotype data were further checked based on Hardy-Weinberg Equilibrium (HWE), minor allele frequencies (MAF), SNP call rate (% of subjects with missing genotypes per SNP) and sample call rate (% of missing SNPs per subject).
