Function: GTPase-activating protein for p21-rac and a phorbol ester receptor. May play an important role in neuronal signal-transduction mechanisms. Tissue specificity: In neurons in brain regions that are involved in learning and memory processes. Disease: Defects in CHN1 are the cause of Duane retraction syndrome type 2 (DURS2) [MIM:604356]. Duane retraction syndrome is a congenital eye movement disorder characterized by a failure of cranial nerve VI (the abducens nerve) to develop normally, resulting in restriction or absence of abduction, adduction, or both, and narrowing of the palpebral fissure and retraction of the globe on attempted adduction. Undiagnosed in children, it can lead to amblyopia, a permanent uncorrectable loss of vision. Similarity: Contains 1 phorbol-ester/DAG-type zinc finger. Contains 1 Rho-GAP domain. Contains 1 SH2 domain. Developmental stage: Increases in amount during brain development coincident with synaptogenesis.