Mucolipidosis II/III (OMIM 252500, 252600 and 252605) is caused by a lack of active enzyme GlcNAc-1-phosphotransferase (GNPTAB, GNPTG, EC 2.7.8.17), which is involved in the phosphorylation step in the synthesis of the mannose 6-phosphate marker present on the lysosomal membrane. This evidence concerns the gene GNPTG and mucolipidosis type II.