Besides their well-established role in PTHS, whole genome association studies show that TCF4 polymorphisms are also associated with schizophrenia (Schizophrenia Psychiatric Genome-Wide Association Study Consortium, 2011; Cross-Disorder Group of the Psychiatric Genomics Consortium, 2013; Kousoulidou et al., 2013), bipolar disorder (Del-Favero et al., 2002; Cross-Disorder Group of the Psychiatric Genomics Consortium, 2013), post-traumatic stress disorder (Gelernter et al., 2019), and major depression disorder (Cross-Disorder Group of the Psychiatric Genomics Consortium, 2013). This evidence concerns the gene TCF4 and major depressive disorder.