TCF4 and major depressive disorder: Changes in the TCF4 gene are linked to the development of many severe neurocognitive disorders such as schizophrenia (Stefansson et al., 2009; Ripke et al., 2014; Doostparast Torshizi et al., 2019), intellectual disability (Kharbanda et al., 2016), post-traumatic stress disorder (Gelernter et al., 2019), and depression (Wray et al., 2018).