The sequence variant c.788C>T in exon 6 changes proline in position 263 to leucine (p.Pro263Leu). Studies in transfected cells show that p.Leu263 is inactive, but processed correctly and localized to the lysosomes. p.Pro263 is located close to the disulphide bridge between p.Cys268 and p.Cys273, and 3d-modelling indicates that p.Pro263Leu may interfere with the formation of this disulphide bridge, causing destabilisation of the enzyme.