Pathology Research International be conﬁrmed through staining for the hepatocytic marker, HepPar-1, and the biliary markers, CK7 and CK19), it can be diﬃcult to diﬀerentiate between ICC and metastasis to the liver from a nonhepatic primary tumor. A few new molecular markers of biliary diﬀerentiation have recently been described [95], but these have not yet been tested for their ability to distinguish ICC from hepatic metastasis. CC generally presents at a late stage, when curative resec- tion is no longer possible. Unfortunately, this malignancy is highly aggressive and resistant to standard chemotherapeutic regimens. Molecular characterization of CC is in the early investigative stages (reviewed in [96]), and useful prognostic molecular markers have not yet been reported. The need for more research in this area, and for the development of targeted therapies and predictive molecular markers, is well recognized. Combined HCC-CC is a rare primary liver tumor characterized by the intimate intermingling of histologically unequivocal HCC and CC components [97]. Most cases show, in addition to typical HCC and CC areas, transitional regions containing immature-appearing cells that express hepatic progenitor markers such as EpCAM. When such transitional regions predominate, the tumor is classiﬁed as a subtype of HCC-CC, “with stem-cell features” [98]. Three histologic patterns of intermediate/stem-cell distribution have been described in lesions of this category, including a pattern sometimes termed cholangiolocarcinoma, formerly classiﬁed as a variant of CC. Although several immunohisto- chemical studies of HCC-CC have conﬁrmed the expression of various hepatic progenitor cell markers in these lesions [99, 100], their classiﬁcation is based on histomorphology, rather than positivity for stem-cell markers per se. The fact that many HPC markers are also expressed by biliary epithelium and the recent ﬁndings on progenitor cell marker expression by HCC greatly complicate this ﬁeld. Thus, although there is evidence that HCC-CC arises from a bipotential hepatic progenitor cell, the histogenesis of this rare neoplasm is still debated (Figure 5). Only a few studies have examined the molecular charac- teristics of HCC-CC. Zucman-Rossi’s group in France used microsatellite-based LOH analysis to assess chromosome stability in 9 CC, 15 HCC-CC, and 3 HCC/CC collision tumors [101]. Combined HCC-CC was found to show a high degree of chromosomal instability, similar to CC and unlike HCC. Investigators in Germany recently reported the results of CGH analysis of 49 HCC, 22 hepatic CC, and 7 HCC-CC cases [102]. In this study, combined HCC- CC was found to resemble both HCC and CC, showing several of the speciﬁc chromosomal gains and losses typical of HCC, while also displaying a high total number of chromosomal imbalances, similar to CC. Woo et al. recently performed gene expression proﬁling of 70 HCC, 13 CC, and 7 HCC-CC [103]. Results showed that the gene expression pattern of HCC was markedly diﬀerent from that of CC, and combined HCC-CC clearly clustered with CC. Further analysis led to the identiﬁcation of a CC gene signature containing many known biliary and hepatic progenitor cell markers. Interestingly, closer examination of the HCC data revealed that a subset of HCC also expressed the CC 11 gene signature; this subset was designated CC-like HCC (CLHCC). Expression of the CC gene signature by HCCs was associated with poor prognosis. However, its prognostic signiﬁcance was found to be independent of previously described poor-prognostic gene signatures such as that of the hepatoblast subtype of HCC. Thus, there are many layers of complexity regarding hepatic progenitor cell and biliary phenotypes in primary liver cancers. Overall, stem-cell and biliary characteristics seem to correlate with increased tumor aggressiveness, resistance to chemotherapy, and poor prognosis. 6. Conclusions In recent years, remarkable progress has been made in elucidating the molecular pathology of hepatic tumors. In the area of benign hepatocellular lesions, advances in our understanding of molecular subtypes have led to a new classiﬁcation system of HCA, with important genotype- phenotype correlations and easy application to routine clinical practice. In the ﬁeld of early hepatic neoplasia, microarray studies of gene expression have identiﬁed sim- ple molecular signatures that can distinguish dysplastic nodules from early HCCs in a background of cirrhosis. Although such expression signatures can be determined by quantitative RT-PCR, the use of immunohistochemistry, which allows the simultaneous assessment of histology and protein expression, is preferred in needle biopsies. Molecular measures of prognosis in patients with HCCs and CC are emerging; in retrospective studies, a variety of molecular techniques have successfully identiﬁed subgroups of HCCs with distinct clinical associations, such as prognosis, risk of recurrence after attempted curative treatment, and response to adjuvant interferon therapy. Although these assays are not yet developed for widespread clinical application, their reﬁnement and use for the stratiﬁcation of patients undergo- ing treatment in clinical trials will lead to further advances in therapeutic options for liver cancer. The current system of classifying hepatic tumors based on histologic architecture and patterns of diﬀerentiation has many limitations. Molec- ular studies have supported a model of hepatic tumorigenesis in which neoplasms recapitulate various lineages and stages of hepatic progenitor cell diﬀerentiation. These discoveries are likely to have an important impact on liver tumor classiﬁcation and to lead to a newer classiﬁcation system, based on histologic and molecular features, with improved prognostic and therapeutic signiﬁcance. References [1] J. F. Buell, H. Tranchart, R. Cannon, and I. Dagher, “Man- agement of benign hepatic tumors,” Surgical Clinics of North America, vol. 90, no. 4, pp. 719–735, 2010. [2] P. Bioulac-Sage, H. Laumonier, A. Rullier et al., “Over- expression of glutamine synthetase in focal nodular hyper- plasia: a novel easy diagnostic tool in surgical pathology,” Liver International, vol. 29, no. 3, pp. 459–465, 2009.