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1. A metabolic disorder characterized by sparse blond hair, long limbs, pectus excavatum, dislocation of lens, failure to thrive, mental retardation, psychiatric disturbances, and thromboembolic episodes; some patients have alleviation of symptoms with pyridoxine whereas others are not responsive; associated with increased urinary excretion of homocystine and methionine. Autosomal recessive inheritance, but carriers have an increased risk of occlusive vascular disease; caused by mutation in the cysthathione beta-synthase gene (CBS) on chromosome 21q. In addition, there are seven other causes of homocystinuria: defect in vitamin B12 metabolism [MIM*277400]; deficiency of N-methylene-tetrahydrofolate reductase [MIM*236250]; selective intestinal malabsorption of vitamin B12 [MIM*261100]; vitamin B12 responsive homocystinuria, cblE type [MIM*236270]; methylcobalamin deficiency, cblG type [MIM*250940], vitamin B12 metabolic defect type 2 [MIM*277410]; and transcobalamin II deficiency [MIM*275350].