Objective  To investigate the relationship between the polymorphism of 5

10-methylenetetrahydrofolate reductase (MTHFR) C677T and the plasma level of homocysteine (Hcy) in Down syndrome (DS) children

and explore the possible mechanisms of mental retardation in DS.   Methods  A case-control study was conducted on 20 G-banded karyotype analysis identified DS patients and 33 matched healthy children from September 2008 to December 2009. The polymorphism in MTHFR C677T was genotyped by PCR-RFLP

then verified by sequence analysis. The plasma level of Hcy was tested by enzyme-linked immunosorbent assay (ELISA). The frequency of MTHFR C677T mutation was compared between DS group and control group. Plasma level of Hcy between children with C/T

T/T genotypes and children with C/C genotype

and between DS group and control group were compared.   Results  The homozygous mutation rate was 11.3% among those 53 Chinese Han children. DS group got remarkable higher mutation rate of MTHFR C677T than control group（P0.05）. DS group had significantly higher T allele frequency （P0.05）. The plasma level of Hcy was remarkable higher in children with C/T and T/T genotypes than in children with C/C genotype（P0.05）. DS group had significantly higher plasma level of Hcy than control group（P0.05）. DS group had remarkable higher incidence of hyperhomocysteinemia than control group（P0.05）.   Conclusion  There is genetic polymorphism in MTHFR C677T among Chinese Han children. The genetic polymorphism in MTHFR C677T is more common in DS group. The mutation of this gene site may lead to lower activity of the key enzyme which takes part in the folic acid metabolism

and cause hyperhomocysteinemia

resulting in mental retardation appeared in DS patients finally.