AIM:To isolate a novel isoform of human HPO(HPO-205)from human fetal liver Marathon-ready cDNA andcharacterize its primary biological function.METHODS:5’-RACE(rapid amplification of cDNA 5’ ends)was used to isolate a novel isoform of hHPO in this paper.The constructed pcDNA~(HPO-205),pcDNA~(HPO)and pcDNAeukaryotic expression vectors were respectively transfectedby lipofectamine method and the stimulation of DNAsynthesis was observed by ~3H-TdR incorporation assay.Proteins extracted from different cells were analyzed byWestern blot.RESULTS:A novel isoform of hHPO(HPO-205)encoding a205 amino acid ORF corresponding to a translatedproduction of 23 kDa was isolated and distinguished fromthe previous HPO that lacked the N-terminal 80 amino acids.The dose-dependent stimulation of DNA synthesis of HepG2hepatoma cells by HPO-205 demonstrated its similarbiological activity with HPO in vitro.The level of MAPK(Mitogen-activated protein kinase)phosphorylation byWestern blot analysis revealed that HPO-205 might have thestronger activity of stimulating hepatic cell proliferation thanthat of HPO.CONCLUSION: A novel isoform of hHPO (HPO-205) was isolated from hepatic-derived cells. The comparison of HPO-205 and HPO will lead to a new insight into the structure and function of hHPO, and provide the new way of thinking to deeply elucidate the biological roles of HPO/ALR. AIM: To isolate a novel isoform of human HPO (HPO-205) from human fetal liver Marathon-ready cDNA and characterize its primary biological function. METHODS: 5’-RACE (rapid amplification of cDNA 5 ’ends) was used to isolate a novel isoform of hHPO in this paper. constructed pcDNA ~ (HPO-205), pcDNA ~ (HPO) and pcDNAeukaryotic expression vectors were respectively transfected by lipofectamine method and the stimulation of DNA synthesis was observed by ~ 3H-TdR incorporation assay. Proteins extracted from different cells were analyzed by Western blot .RESULTS: A novel isoform of hHPO (HPO-205) encoding a205 amino acid ORF corresponding to a reproduced production of 23 kDa was isolated and distinguished from the previous HPO that lacked the N- terminal 80 amino acids. dose- dependent stimulation of DNA synthesis of HepG2 hepatoma by HPO-205 demonstrated their similar biological activity with HPO in vitro. The level of MAPK (Mitogen-activated protein kinase) phosphorylation by Western blot analysis revealed that H PO-205 might have thestronger activity of stimulating hepatic cell proliferation thanthat of HPO. CONCLUSION: A novel isoform of hHPO (HPO-205) was isolated from hepatic-derived cells. The comparison of HPO-205 and HPO will lead to a new insight into the structure and function of hHPO, and provide the new way of thinking to deeply elucidate the biological roles of HPO / ALR.