Mucopolysaccharidosis IVA (MPS IVA) is caused by the deficiency of the lysosomal enzyme N-acetylgalactosamine-6-sulfate sulfatase encoded by the GALNS gene on chromosome 16. We describe, in detail, the clinical phenotype of five patients from three unrelated Finnish families and have characterized the disease-causing mutations in GALNS. Genotypes of the patients are D60N/A291T, D60N/W230X, and D60N/1374delT. Mutation 1374delT introduces premature termination of GALNS. Cells over-expressing the novel mutation W230X and A291T had no residual GALNS activity, whereas D60N gave 12.2% residual activity compared with the wild type. Co-transfection of D60N/A291T and D60N/W230X showed 5.5% and 6.7% of wild type activity, respectively. The precursor proteins of D60N and A291T were observed at 55 kDa and 57 kDa, respectively, whereas there was no detectable band in cells over-expressing W230X. At 55 degrees C, the mutant protein showed lower thermostability than the wild type protein at pH 3.8 and 7.0. The tertiary structural model of the GALNS protein revealed that aspartic acid at position 60 is located on the surface of the molecule, away from the active site. This makes it unlikely that the enzymatic function of the protein with D60N is severely impaired. On the other hand, A291 and W230 are localized near the active site. The molecular characteristics of the D60N mutation explain the attenuated clinical phenotype of the patients.