Nogo-B, also known as Reticulon 4B, plays important roles in vascular injuries. Its function in the liver is not understood. The aim of this study was to characterize Nogo-B in liver fibrosis and cirrhosis. Nogo-B distribution was assessed in normal and cirrhotic human liver sections. We also determined the levels of liver fibrosis in wild-type (WT) and Nogo-A/B knockout (NGB KO) mice after sham operation or bile duct ligation (BDL). To investigate the mechanisms of Nogo-B's involvement in fibrosis, hepatic stellate cells were isolated from WT and NGB KO mice and transformed into myofibroblasts. Portal pressure was measured to test whether Nogo-B gene deletion could ameliorate portal hypertension. In normal livers, Nogo-B expression was found in nonparenchymal cells, whereas its expression in hepatocytes was minimal. Nogo-B staining was significantly elevated in cirrhotic livers. Fibrosis was significantly increased in WT mice 4 weeks after BDL compared with NGB KO mice. The absence of Nogo-B significantly reduced phosphorylation of Smad2 levels upon transforming growth factor b (TGF-b) stimulation. Reconstitution of the Nogo-B gene into NGB KO fibroblasts restored Smad2 phosphorylation. Four weeks after BDL, portal pressure was significantly increased in WT mice by 47%, compared with sham-operated controls (P 5 0.03), whereas such an increase in portal pressure was not observed in NGB KO mice (P 5 NS). Conclusion: Nogo-B regulates liver fibrosis, at least in part, by facilitating the TGFb/Smad2 signaling pathway in myofibroblasts. Because absence of Nogo-B ameliorates liver fibrosis and portal hypertension, Nogo-B blockade may be a potential therapeutic target in fibrosis/cirrhosis. (HEPATOLOGY 2011;53:1306-1315) N ogo-B, also known as Reticulon 4B, is a member of the reticulon (Rtn) family of proteins that are primarily localized to the endoplasmic reticulum. In mammalian cells, there are four Rtn genes, Rtn-1, -2, -3, and -4, and each gene encodes multiple isoforms. For Rtn 4, there are three gene products: Nogo-A, -B, and -C. Nogo-A and -C are highly expressed in the central nervous system, with Nogo-C also present in the skeletal muscle. 1,2 Nogo-A is known to inhibit axonal growth and repair, whereas the function of Nogo-C is not well known.