TCF4 and Pitt-Hopkins syndrome: The importance of TCF4 in human nervous system development is underscored by the association of a TCF4 allele with schizophrenia [21] and identification of TCF4 haploinsufficiency as the cause for Pitt-Hopkins syndrome (OMIM 610954), a rare disease featuring mental retardation, hyperventilation and seizures [22]–[24].