Mucolipidosis II and III are autosomal recessive disorders due to mutations in the GNPTAB and GNPTG genes encoding the alphabeta- and gamma-subunits of the GlcNAc-1-phosphotransferase, respectively. This protein has a subunit structure of alpha(2)beta(2)gamma(2) and initiates the first step of tagging lysosomal enzymes with mannose-6-phosphate (M6P). In the present study, we screened four MLII and three MLIII cell lines for mutations in GNPTAB and GNPTG. Nine novel mutations in GNPTAB and two previously reported mutations in GNPTAB and GNPTG were identified. By using anti-peptide antibodies against the alpha- and beta-subunits, we show that mutations in the gamma-subunit affected the assembly and intracellular distribution of the alpha- and beta-subunits. Furthermore, the biochemical phenotypes of MLII and MLIII fibroblasts can be corrected by transfection with wild-type cDNA expression constructs encoding the alpha/beta- and gamma-subunits of GlcNAc-1-phosphotransferase, respectively.