Major depression (MD) is a common psychiatric disorder with a lifetime prevalence rate of 15–17% (95% CI) (Ebmeier et al., 2006). It is not only a potentially fatal disease with about 2% of patients committing suicide (Bostwick and Pankratz, 2000)but also one of the leading causes worldwide for loss in work productivity (Ebmeier et al., 2006; Ustun et al., 2004). Current treatments are indispensable but their clinical efficacy is still unsatisfactory, as reflected by high rates of treatment resistance and side effects (Fava and Rush, 2006). Identification of mechanisms causing depression is pertinent for discovery of better antidepressants. The heritability of this disorder has been estimated to range from 34–42% (95% CI) (Ebmeier et al., 2006) and several attempts to identify susceptibility genes by linkage and candidate gene approaches have been undertaken. In candidate gene studies, BDNF, SLC6A4, ACE, P2RX7, 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
