“…Studies of model organisms suggested epistasis or gene-gene interactions to be a common phenomenon [13,14,15,16], and a number of gene-gene interactions have been reported in gene mapping studies in animals, plants, and other model organisms [17,18,19,20]. However, gene-gene interactions have proven difficult to find in humans [21,22], mainly due to low statistical power caused by the small effect size, the low minor genotype frequency of the multiple-SNP combinations, the large combinatorial number of interaction tests required [14,23], and the lack of control of environmental conditions. Hence, in order to improve the power of detection of gene-gene interactions in human GWAS, many approaches have been developed to prioritize candidate genes or SNPs using biological knowledge from established GWAS hits [6,24], protein-protein interactions (PPIs) [25,26], and pathway information [27].…”