AIM: Over 90% of drugs are metabolized by the cytochrome P-450 (CYP) family of liver isoenzymes. The most important enzymes are CYP1A2, 3A4, 2C9/19, 2D6 and 2E1. Although CYP2D6 accounts for ＜2% of the total CYP liver enzyme content, it mediates metabolism in almost 25% of drugs. In order to study its enzymatic activity for drug metabolism, its cDNA was cloned and a HepG2 cell line stably expressing CYP2D6 was established.METHODS: Human CYP2D6 cDNA was amplified with reverse transcription-polymerase chain reaction (RT-PCR)from total RNA extracted from human liver tissue and cloned into pGEM-T vector, cDNA segment was identified by DNA sequencing and subcloned into a mammalian expression vector pREP9. A cell line was established by transfecting the recombinant plasmid of pREP9-CYP2D6 to hepatoma HepG2 cells. Expression of mRNA was validated by RT-PCR.Enzyme activity of catalyzing dextromethorphan O-demethylation in postmitochondrial supant (S9) fraction of the cells was determined by high performance liquid chromatography (HPLC).RESULTS: The cloned cDNA had 4 base differences, e.g.100 C→T, 336 T→C, 408 C→G and 1 457 G→C, which resulted in P34S, and S486T amino acid substitutions, and two samesense mutations were 112 F and 136 V compared with that reported by Kimura et al(GenBank accession number: M33388). P34S and S486T amino acid substitutions were the characteristics of CYP2D6*10 allele. The relative activity of S9 fraction of HepG2-CYP2D6*10 metabolized detromethorphan O-demethylation was found to be 2.31±0.19 nmol.min-1.mg-1 S9 protein (n=3), but was undetectable in parental HepG2 cells.CONCLUSION: cDNA of human CYP2D6*10can be successfully doned. A cell line, HepG2-CYP2D6*10, expressing CYP2D6*10 mRNA and having metabolic activity, has been established.