<p><b>OBJECTIVE</b>Mutations of the iduronate-2-sulfatase (IDS) gene is the ultimate cause of Hunter syndrome. Clarification of the nature of mutations will create a necessary premise for prenatal gene diagnosis. A mucopolysaccharidosis (MPS) type II patient and his parents from an ethnic minority in Yunnan province were studied to identify their possible mutation in IDS gene to establish the basis for prenatal gene diagnosis.</p><p><b>METHODS</b>The patient was a boy, 6 years and 10 months old. Urine glycosaminoglycans (GAGs) assay was used for preliminary diagnosis of the patient and his parents with the disease. The three related persons' DNA was extracted and the concentration and purity of the DNA were measured after the urine test results confirmed the diagnosis. Polymerase chain reaction-denaturing high performance liquid chromatography (PCR-DHPLC) analysis was performed to detect the position of the mutation around the hot spots of mutation in exon 9, 3, 8 of the IDS gene. DNA bidirectional direct sequencing was applied to analyze the mutation detected by PCR-DHPLC.</p><p><b>RESULTS</b>The results of GAGs test showed that in the child with MPS, dermatan sulfate (DS) was positive (+++), heparan sulfate (HS) (+++), chondroitin sulfate (CS) and keratan sulfate (KS) were negative (-); while in his parents none of DS, HS, CS and KS was positive. Abnormal peaks in exon 9 of IDS gene shown by PCR-DHPLC were found in the patient. His mother had heterozygotic peaks. A new frame-mutation (1343-TT) in exon 9 of IDS gene of this patient was confirmed by DNA sequencing. The position where mutation occurred was inside codon 407 (TTT), that means two "T" deleted at position 1343 base pair (1343-TT) in cDNA of the IDS gene, caused a new frame-mutation. It caused elongation of the amino acid chain to a terminal codon TGA at position 429. Thus the peptide chain was shortened from 550 to 428 amino acids. The patient is a hemizygote of the mutation and his mother is a heterozygote.</p><p><b>CONCLUSION</b>A new frame-mutation (1343-TT) on the IDS gene was identified in this study. The patient is a hemizygote and his mother is a heterozygote. The mutation (1343-TT) resulted in loss of 122 amino acids, which probably caused seriously decreased enzyme activity of IDS, and the authors speculate that this mutation may be the pathological basis of the disease. So, if the mother becomes pregnant again, a prenatal gene diagnostic test for the same mutation should be performed. Furthermore, PCR-DHPLC followed by DNA sequencing are effective methods for diagnosis, including prenatal diagnosis of MPS II.</p>