TCF4 and Intellectual disability: Variants in TCF4 have been associated with schizophrenia, autism and intellectual disability and TCF4 haploinsufficiency causes the neurodevelopmental disorder Pitt–Hopkins syndrome (PTHS) (OMIM 610954) (Stefansson et al., 2009; Steinberg et al., 2011; De Rubeis et al., 2014; Zweier et al., 2007; Amiel et al., 2007).