Aspartylglucosaminuria is an autosomal recessive (Gene AGA) lysosomal storage disorder that involves the central nervous system and causes skeletal abnormalities as well as connective tissue lesions. Infants with aspartylglucosaminuria appear healthy at birth, and development is typically normal throughout early childhood. The first sign of this condition, evident around the age of 2 or 3, is usually delayed speech. Mild intellectual disability then becomes apparent, and learning occurs at a slowed pace. Intellectual disability progressively worsens in adolescence. Most people with this disorder lose much of the speech they have learned, and affected adults usually have only a few words in their vocabulary. Adults with aspartylglucosaminuria may develop seizures or problems with movement. Mutations in the AGA gene cause aspartylglucosaminuria. The AGA gene encodes an enzyme called aspartylglucosaminidase. This enzyme is active in lysosomes where it helps break down complexes of oligosaccharides attached to certain glycoproteins. AGA gene mutations result in the absence or shortage of the aspartylglucosaminidase enzyme in lysosomes, preventing the normal breakdown of glycoproteins. As a result, glycoproteins can build up within the lysosomes which disrupt the normal functions of the cell and can result in cell death. Aspartylglucosaminuria is estimated to affect 1 in 18,500 people in Finland. This condition is less common outside of Finland.