TCF4 and major depressive disorder: Intriguingly, recent studies suggest that in addition to TCF4 haploinsufficiency, increased TCF4 dose is also a risk factor for disturbed cognitive development as a TCF4 duplication has been described in a patient with developmental delay (Talkowski et al., 2012) and a partial duplication in a patient with major depressive disorder (Ye et al., 2012).