Hyperhomocysteinemia is an independent risk factor of cardiovascular diseases and birth defects. One of the important factors causing hyperhomocysteinemia is decrease of 5,10-Methylenetetrahydrofolate reductase. Human and rat MTHFR cDNAs with RT-PCR were isolated, a prokaryodytic expression vector containing human MTHFR cDNA was constructed, and human MTHFR protein was expressed inE. coli. It was also found that the expression of rat MTHFR could be promoted by IL-1 and homocysteine.