In our study we describe the clinical symptoms and the results of biochemical and molecular analyses in eight patients with CDG syndrome type I. Methods: The nonglycosylated and hypoglycosylated transferrins were measured in serum using turbidimetric immunoassay and isoelectric focusing. The genes for phosphomannomutase 2 (PMM2) and
-1,3-glukosyltransferase (ALG6) were sequenced. Results: In all patients, increasedamounts of hypo- and nonglycosylated transferrins were observed. The disease is manifested in infancy with failure to thrive, muscle hypotonia, epilepsy, microcephaly and psychomotor retardation. In most patients, strabismus and inverted nipples were present and hypoplasia of the cerebellum was found in six patients. One boy had cyclic pericardial effusion and another boy was operated on ventricular septal defect. In most patients, low concentration of antitrombin III, factor XI and protein C and S were found. The activities of -glucuronidase, -mannosidase, -hexosaminidase and arylsulfatase A in serum were increased and the activities of
-mannosidase and -mannosidase in leucocytes were decreased in comparison with controls. Molecular analyses revealed that two siblings were compound heterozygotes for mutations G422A and C357C in gene for phosphomannomutase 2 (PMM2) and also homozygotes for polymorphism IVS5+19 C  T and heterozygotes for polymorphism IVS+22 T  A. One girl is a compound heterozygote for missence mutations G422A and C338T in the gene forPMM2. Three other patients are heterozygotes for mutations G422A, the second mutation was not found. In one child, a heterozygous missense mutation T911C was found in the gene for ALG6. Conclusion: The prognosis of children withCDGsyndrome type I is unfavourable.Enzymatic and/ormolecular analyses are necessary for genetic counseling and for the prenatal diagnosis in affected families.

        Key words: CDG syndrome type Ia, psychotomotor retardation, cerebellar hypoplasia