Mucopolysaccharidosis III (Sanfilippo syndrome) was described in 1963 by a pediatrician named Sylvester Sanfilippo (J. Pediat. 63: 837-838, 1963, no reference). Mucopolysaccharidosis type IIID (MPS IIID, Sanfilippo syndrome D, MIM:252940) is an autosomal recessive genetic disorder due to the loss of N-acetyl-D-glucosamine 6-sulfatase (GNS; MIM:607664), that hydrolyses the 6-sulfate groups of the N-acetyl-D-glucosamine 6-sulfate units of the glycosaminoglycans (GAGs) heparan sulfate and keratan sulfate. GNS is localized to chromosome 12q14 and has 14 exons spanning 46 kb (Robertson et al. 1988, Mok et al. 2003). Loss of enzyme activity leads to lysosomal accumulation and urinary excretion of heparan sulfate and N-acetylglucosamine 6-sulfate residues (Mok et al. 2003). Keratan sulphate does not accumulate in MPS IIID, as beta-linked N-acetyl-D-glucosamine 6-sulphate can be cleaved by beta-hexosaminidase A (Kresse et al. 1980). This disorder is characterized by progressive mental deterioration but only moderate physical abnormalities and death duing the second or third decade of life, presenting a phenotype similar to MPSIIIA (Jones et al. 1997, de Ruijter et al. 2011).