A minority of patients with atypical Rett syndrome have mutations in the cyclin-dependent kinase-like 5 (CDKL5), Forkhead box protein G1 (FOXG1), myocyte-specific enhancer factor 2C (MEF2C), and transcription factor 4 (TCF4) genes [165]. Here, TCF4 is linked to atypical Rett syndrome.