The diagnosis of MPS I issupported by molecular and biochemical assays to confirm the association of the altered catalytic activity of IDUA with the clinicalphenotype, for which enzymatic activity assays are performed with the synthetic substrate 4-methylumbelliferyl a-L-iduronide (4-MUI).To date, with molecular and enzymatic activity studies of rare or new variants of IDUA, it is not possible to predict the phenotype thata patient will develop. Here, IDUA is linked to Scheie syndrome.