TCF4 and bipolar disorder: Mutations and common polymorphisms within TCF4 have been implicated in several diseases including bipolar disorder, schizophrenia, Pitt-Hopkins Syndrome (OMIM # 610954), and Sonic Hedgehog (SHH) medulloblastoma tumours (Blake et al., 2010; Blanluet et al., 2019; Forrest et al., 2012, 2018; Hellwig et al., 2019; Rannals and Maher, 2017; Roussos, 2012; Sweatt, 2013; Xia et al., 2018).