The sequence variant c.222C>A in exon 2 changes aspartate in position 74 to glutamate (p.Asp74Glu). Studies in transfected cells show that p.Glu74 is inactive, but processed correctly and localized to the lysosomes. p.Asp74 is located in the active site where it is directly involved in coordination of the cofactor Zn2+, and 3d-modelling indicates that the substitution changes the metal binding site and thus the catalytic properties of the enzyme. Hence, p.Asp74Glu is an active site mutant.