Sanfilippo syndrome is a genetic error of metabolism. It is a mucopolysaccharidoses, an MPS disorder, MPSlll. Mucopolysaccharides are long chains of sugar molecules, which are used in building connective tissues. When the body is finished using these molecules, it breaks them down, with enzymes, and disposes of them. Children with Sanfilippo syndrome are missing or are deficient in the enzyme to break down the molecules. Instead, the body stores these molecules in the cells. This storage causes progressive damage.