Mucolipidosis type II (MLII; MIM #252500), the most severe form of the disease, is caused by mutations in the GNPTAB gene encoding the membrane-bound precursor of α- and β-subunits of GlcNAc-1-phosphotransferase (EC 2.7.8.17) (Tiede et al., 2005). Here, GNPTAB is linked to mucolipidosis type II.