Mucopolysaccharides are long chains of sugar molecules which are used in building connective tissue.
Typically, when the body is finished using these sugar molecules, it breaks them down with enzymes and disposes of them. Children with Sanfilippo syndrome, however, are missing the enzyme to breakdown the molecules and instead store them in cells in their body. The storage of these molecules then causes progressive damage. As a result, infants may not show signs of the disorder, but as the child grows and more cells become damaged, the symptoms become more obvious and worsen. There are four different enzyme deficiencies that cause Sanfilippo syndrome, therefore, the syndrome is classified as being either type A,B,C, or D. Type A is caused by a deficiency of the enzyme known as heparan sulfatase and is the most severe type with earlier onset and earlier death than the others.