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Some disorders that compromise the nervous system reflect single-gene mutations in homeobox-like transcription factors. Aniridia (characterized by loss of the iris in the eye and mild mental retardation) and Waardenburg's syndrome (characterized by craniofacial abnormalities, spina bifida, and hearing loss) are the result of mutations in the PAX6 and PAX3 genes, respectively, both of which encode transcription factors.