TCF4 and Intellectual disability: Heterozygote loss-of-function mutations of TCF4 before exon 7 have been linked to non-specific intellectual disability [2–4], while heterozygote disrupting mutations after exon 9 have been causally linked to Pitt-Hopkins Syndrome (PTHS, MIM #610954), a neurodevelopmental disorder characterized by distinctive facial features, moderate to severe intellectual disability, autistic behavior, intermittent breathing abnormalities, seizures [5–7].