The ARX gene is involved in the development of GABAergic interneurons in the 
forebrain. Loss-of-function mutations, such as nonsense or frameshifts mutation, 
of ARX cause a group of brain malformations, such as hydranencephaly, 
lissencephaly, and agenesis of the corpus callosum, while expansion mutations of 
the polyalanine tracts of ARX, supposed to be gain-of-function mutations, result 
in a non-malformation group, such as non-syndromic mental retardation, mental 
retardation with dystonia, West syndrome, and Ohtahara syndrome. A variety of 
phenotypes caused by pleiotropic mutations of the ARX gene are considered to 
share a common pathological mechanism connected with the structural and 
functional disturbance of interneurons, designated as 'interneuronopathies'. We 
identified the second gene responsible for Ohtahara syndrome, STXBP1, which is 
essential for synaptic vesicle release. Molecular studies of the diseases will 
reveal the relationships between the structure and function of the brain. It is 
indispensable to clarify the etiology of hereditary diseases and identify new 
approaches to treatment.