Mutations of human jagged 1 (JAG1) gene are responsible for Alagille Syndrome (AGS), whose 2 main symptoms are intrahepatic bile duct hypoplasia and pulmonary stenosis. We examined the JAG1 mutation in extrahepatic biliary atresia (EHBA), which is similar in phenotype to AGS, although a different pathogenesis is suggested. In 102 cases of EHBA, 9 missense mutations were detected, including 2 intrafamilial expressions in the propositus and an aunt of one family. These mutations were all missense and sporadic except for those of this particular family. The JAG1 gene mutations were generally found in severely ill patients subjected to liver transplantation at less than 5 years of age. None of the 9 cases of EHBA revealed any of the 5 major symptoms of AGS nor any identical pathological findings after 3 years of follow-up. Our cases were clearly separated from AGS by pathological findings and clinical features, and could be diagnosed as EHBA and not as atypical AGS. The increase of interleukin 8 (IL-8) production induced by tumor necrosis factor α (TNF-α) in Huh 7 cells was suppressed by the coexistence of JAG1 protein. We examined the different influences between wild-type cells and the 3 kinds of mutants detected in EHBA on Huh 7 cells and found that 2 of 3 mutants showed about half of the repressed activity compared with that of wild type. In conclusion, these results suggest that the JAG1 gene abnormality may be an aggravating factor in EHBA.