TCF4 and neurodevelopmental disorder: TCF4 is a dosage-sensitive protein: too little expression causes neurodevelopmental disorders, and too much expression may be linked to schizophrenia (Brennand et al., 2011; Brzózka et al., 2010; Forrest et al., 2012; Quednow et al., 2014; Sepp et al., 2012; Wirgenes et al., 2012).