We investigated the expression of the mitochondrial ATPase6 gene whose product is active in oxidative phosphorylation (OXPHOS), and compared it to the expression of Tfam, an important regulator of the transcription and replication of mtDNA. Our aim was to examine a possible relation between mitochondrial gene expression and Down syndrome. The expression of ATPase6 and Tfam was analyzed by RT-PCR amplification of the mRNA in cultured amniocytes from Down syndrome and normal fetuses. The band intensities obtained were normalized against those of HPRT. The Down syndrome fetuses were found to have lower ATPase6 and Tfam expression than the normal fetuses. This finding suggests that mitochondrial dysfunction resulting from decreased ATPase6 and Tfam expression during meiotic oocyte maturation of oocytes might affect ATP generation and cause the nondisjunctional error. Hence this study suggests that mitochondrial dysfunction may be associated with the developmental mechanism of Down syndrome.