Aspartylglycosaminuria (AGU) is a classical lysosomal storage disorder caused by defective activity of the lysosomal hydrolase aspartylglucosaminidase (AGA), resulting in the accumulation of abnormal degradation products in lysosomes. AGA, encoded by a gene on chromosome 4q34-35, normally cleaves the bond between N-asparagine and N-acetylglucosamine in Asn-linked glycoproteins. Neuronal accumulation of storage substances gives rise to a slowly progressive global developmental delay, with first presentation usually between two and four years of age. During the preclinical phase AGU-children often suffer from prolonged upper respiratory infections. The developmental profile of both motor and cognitive skills lags steadily behind that of normal children, and at the puberty AGU patients are mildly or moderately mentally retarded. With increasing age overall performance further declines; the life span of severely retarded individuals is 45 to 50 years. Brain MRI shows abnormalities in the differentiation between gray and white matter and signs of delayed myelination. Diagnosis of AGU can be made by detecting abnormal glycoasparagines in urine, or by assaying the activity of the AGA enzyme in peripheral blood leukocytes or by a genetic mutation test. Gene tests and enzyme assays also allow fetal diagnosis with a chorionic villus biopsy, but carrier identification is possible only with a gene test