Recently, we found a region A (a hepatocyte nuclear factor (HNF)-4-binding site from nucleotides -701 to -684) and a region B (an HNF-1-binding site from nucleotides -682 to -666) as cis-acting elements necessary for the transcriptional activation of the human dihydrodiol dehydrogenase (DD)4 gene in human hepatoblastoma HepG2 cells, which express DD4 mRNA.