TPH2, PDE9A, PDE11A, DISC1, and GRIK3 have been reported to be associated with the disease (Levinson, 2006). Only few of these initial reports have been confirmed by subsequent studies or in meta-analyses. In the last years, the first genome-wide association (GWA) case-control studies in MD were published. None reported genome-wide significant results and their top hits were difficult to be replicated (Lewis et al., 2010a; Muglia et al., 2008; Rietschel et al., 2010; Shi et al., 2010; Sullivan et al., 2009; Wray et al., 2010). Phenotypic diversity and genetic heterogeneity as well as a considerable environmental contribution inherent to MD have been considered to represent major obstacles for the identification of causative variants.
