Hepatic stimulating substance (HSS) was first isolated from weanling rat liver in 1975 and found to stimulate hepatic DNA synthesis bothin vitroandin vivo. Since then, mammalian and human HSS have been investigated for their potential to treat hepatic diseases. However, the essential nature in composition and structure of HSS remain puzzling because HSS has not been completely purified. Heating, ethanol precipitation, and ion-exchange chromatographies had been carried out to isolate the protein with specific stimulating activity from newborn calf liver, and [3H]thymidine deoxyribose (TdR)/bromodeoxyuridine (BrdU) incorporation and carboxyfluorescein diacetate succinimidyl ester (CFSE)-based proliferation assay to determine the bioactivityin vitroandin vivo. We report the purification of a novel 30-kDa protein from a crude extract of calf liver HSS. This protein is a member of the leucine-rich acidic nuclear protein family (LANP) and has been named hepatopoietin Cn (HPPCn). Studies of partially hepatectomized (PH) mice show that levels of HPPCn messenger RNA (mRNA) increase after liver injury. Furthermore, the recombinant human protein (rhHPPCn) was shown to stimulate hepatic DNA synthesis and activate signaling pathways involved in hepatocyte proliferationin vitroandin vivo.Conclusion:HPPCn is a novel hepatic growth factor that plays a role in liver regeneration.