TCF4 and Pitt-Hopkins syndrome: Understanding the roles of Da in the nervous system is important because its human ortholog, E-protein TCF4, is implicated in various nervous system disorders—mutations in TCF4 lead to an autism spectrum disorder Pitt–Hopkins syndrome (PTHS), and its common gene variants are linked to schizophrenia (Amiel et al., 2007; Brockschmidt et al., 2007; Zweier et al., 2007; Doostparast Torshizi et al., 2019).