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Hyperlysinaemia is a lysine metabolism disorder characterised by elevated levels of lysine in the cerebrospinal fluid and blood. Variable degrees of saccharopinuria are also present. The prevalence is unknown. The disorder was first reported in individuals with neurological problems and intellectual deficit. However, subsequent studies conducted to avoid ascertainment bias identified pronounced hyperlysinaemia in otherwise asymptomatic individuals, suggesting that isolated hyperlysinaemia is not associated with a clinical phenotype. One mother with hyperlysinaemia gave birth to a normal baby. Hyperlysinaemia is transmitted as an autosomal recessive trait. It is caused by a deficiency of the bifunctional enzyme alpha-aminoadipate semialdehyde synthase (AASS gene, 7q31.3). This enzyme has both lysine-ketoglutarate reductase (LKR) and saccharopine dehydrogenase (SDH) activity, and catalyses the first two steps of lysine degradation. In hyperlysinaemia, both enzymatic functions of alpha-aminoadipate semialdehyde synthase are defective. A low lysine diet may be of benefit in some cases.