The CSE1L/CAS protein (CAS) is a Ran-binding protein with a function as nuclear transport (export) factor. Like recently observed for ran and other ran-binding proteins, CSE1L/CAS simultaneously plays a role in the mitotic spindle checkpoint, which assures genomic stability during cell division. This checkpoint is frequently disturbed in neoplasias of various origin, including hepatic tumors. We have evaluated by immunohistology the expression of CAS in adult and embryonic liver, hepatitis, and in liver hyperplasias. Normal hepatocytes revealed no CAS expression while embryonic liver showed strong expression in all parenchymal cells. Bile ducts stained positive with anti-CAS antibodies, and strong CAS expression was also detected at the interface between bile ducts and hepatocytes under conditions associated with regenerative proliferation. The localization of these CAS expressing cells correlated with the distribution of putative liver stem-cells. In active viral (but not in inactive) hepatitis, strong hepatocytal CAS expression correlates in site and intensity with degree of inflammation. Neoplastic liver demonstrated different degrees of CAS expression: no remarkable expression in adenomas, moderate expression in a narrow rim of hepatocytes and in periseptal cholangiolar proliferations in focal nodular hyperplasia, and strong CAS expression in hepatocellular carcinoma. Less differentiated tumors stain stronger than well differentiated. Cholangio-cellular carcinomas show even stronger CAS expression than hepatocellular carcinomas. Our observation of strong expression of CAS in liver cells that are committed for proliferation among them possibly liver stem cells, and in liver neoplasms, is consistant with the fact that CAS functions not solely as a nuclear transport factor but that it is also essential for cell proliferation, particularly for the mitotic spindle checkpoint. Interestingly, genomic instability is frequently observed in hepatic tumors which we have shown here to express large amounts of CAS. Since the degree of CAS-expression correlates with the grade of tumor dedifferentiation, we suggest that CAS should also be further investigated as prognostic marker for hepatic neoplasms.