Transcription factor 4 (TCF4), a member of the basic helix–loop–helix family (bHLH) of proteins, has been causally linked to intellectual disability and neuropsychiatric diseases such as schizophrenia, major depression, or autism spectrum disorder (Betancur, 2011; Vissers et al, 2016; Willsey et al, 2018). This evidence concerns the gene TCF4 and schizophrenia.