DISC1 and major depressive disorder: Transgenic disrupted-in-schizophrenia-1 (Disc1)-mutant mice, which resemble an ultra-rare mutation with high penetrance for schizophrenia and major depressive disorder (Blackwood et al., 2001), show cognitive deficits, thus rendering Disc1-mutant mice a prime candidate to investigate the circuit mechanisms of a psychiatric disease (Koike et al., 2006; Kvajo et al., 2008).