The repeated test of enzyme activities confirmed isolated I2S deficiency and normal levels of other lysosomal enzymes together with normal genotyping of SUMF1 gene (sulfatase modifying factor 1) (in all the three cases reported, the IDS activity in peripheral blood leukocytes was determined by method previously described by Voznyi et al. [13]). Here, SUMF1 is linked to mucopolysaccharidosis type 2.