TCF4 and Global developmental delay: TCF4 has gained major attention following the discoveries that heterozygote loss-of-function mutations in TCF4 cause Pitt-Hopkins syndrome (PTHS) [2, 3]—a neurodevelopmental disorder characterized by early onset developmental delay, moderate to severe intellectual disability, autistic behavior, intermittent breathing abnormalities, seizures, and distinctive facial features—and that single nucleotide polymorphisms (SNPs) in potentially regulatory regions of TCF4 are associated with an increased risk of schizophrenia [4–6].