Mucolipidosis type II (ML II; I-cell disease) and mucolipidosis III (ML III; pseudo Hurler polydystrophy) are autosomal recessively inherited disorders caused by a defectiveN-acetylglucosamine 1-phosphotransferase (phosphotransferase). The formation of mannose 6-phosphate markers in soluble lysosomal enzymes is impeded leading to their increased excretion into the serum, to cellular deficiency of multiple hydrolases, and lysosomal storage of non-digested material. Phosphotransferase deficiency is caused by mutations inGNPTAandGNPTGencoding phosphotransferase subunits. Here we report on an adolescent with progressive joint contractions and other signs of mucolipidosis II who survived to the age of 14 years. Impaired trafficking of lysosomal enzymes cathepsin D and β-hexosaminidase in metabolically labeled fibroblasts was documented. Mutations in theGNPTGgene and alterations in theGNPTGmRNA level were not detected. A different electrophoretic mobility of the 97 kDa GNPTG dimer suggested posttranslational modification abrogating the compartmentalization of GNPTG in the Golgi apparatus. A nucleotide substitution in theGNPTAgene (c.3707A>T) was identified altering the predicted C-terminal transmembrane anchor of the phosphotransferase subunit. The data demonstrate that defective GNPTA not only impairs lysosomal enzyme targeting but also the availability of intact GNPTG required for phosphotransferase activity and assembly of subunits. © 2006 Wiley-Liss, Inc.