more than 1,000,000 genetic variants (in this case single nucleotide polymorphisms, SNPs), distributed across the entire genome are genotyped using a microarray platform and (individually) tested for association with a trait or disorder. The selection of SNPs is mostly based on the distribution of linkage disequilibrium (LD) across the genome, as identified by the HapMap project (Frazer et al. 2007; The International HapMap Consortium 2005). This selection is sufficient to obtain information about most of the frequent genetic variation in the genome (Manolio et al. 2008; Pearson and Manolio 2008), especially for the newer platforms, which include around 1,000,000 or more SNPs. In the short time of their existence, GWAS have already led to the identification of more than 100 genetic variants significantly associated with about 40 different traits and diseases, including a number of psychiatric ones (e.g. Ferreira et al. 2008; O’Donovan et al. 2008; O’Donovan et al. 2009; Sklar et al. 2008). Although highly successful, design-wise (for an overview of GWAS approaches see Neale and Purcell 2008), the GWAS method has not been exploited to the fullest, given its high financial costs (sample sizes of several thousands to ten-thousands may be needed, see for a recent discussion of
