Collybistin, also known as ARHGEF9, is an ~60kD cytoplasmic guanine nucleotide exchange factor. It is expressed in the brain and is upregulated at the onset of neuronal differentiation. Collybistin associates with Neuroligin­2, Gephyrin, and GABA receptor subunits to form a complex which is required for GABAergic synaptogenesis as well as hippocampal synaptic plasticity. Mutations in Collybistin are associated with a variety of neurological disorders including epilepsy, anxiety, aggression, mental retardation, and hyperplexia. Human Collybistin contains an SH3 domain (aa8­67), a DH domain (aa103­287), and a Pleckstrin homology domain (aa318­426). Within aa390­516, human Collybistin shares 97% and 57% aa sequence identity with mouse and rat Collybistin, respectively.