Behçet's disease is a multisystem inflammatory disorder with unknown etiology. It has been shown that the titer of plasma complement component 9 (C9) is a good indicator of the disease activity. Therefore, the involvement of C9 in the pathogenesis of Behçet's disease has been suggested. We report a case of Behçet's disease associated with complete C9 deficiency (C9D) carrying the homozygous nonsense mutation at Arg-95 of C9 (R95X). The patient presented the typical characteristics of Behçet's disease, such as uveitis, recurrent oral aphthae and genital ulcers, and arthritis, suggesting that C9 does not play an essential role in the pathogenesis of Behçet's disease.
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