Pathology Research International as well as progenitor cells and biliary epithelium [14], shows a distinct pattern of expression in HCA [15] that distinguishes the latter from FNH in most cases (Figure 2). In our experience, although occasional needle biopsies remain diagnostically challenging even after the combined use of immunohistochemical stains for CK7 and CK19, this analysis is usually quite helpful in the diﬀerential diagnosis of FNH and HCA [13, 16]. Immunohistochemical staining for the cytoplasmic enzyme, glutamine synthetase (GS), has also been proposed as a technique for diﬀerentiating between FNH and HCA [2]. GS is an enzyme that combines glutamate with ammonia to produce the amino acid glutamine. In normal liver, GS expression is limited to the centrilobular hepatocytes that directly border central veins, and its most important function is to aid in ammonia detoxiﬁcation [17]. In recent years, the Wnt signaling pathway has been identiﬁed as a critical regulator of zonation in the liver [18]. Beta- catenin activation in perivenular hepatocytes, presumably the result of Wnt signaling from central veins, drives the expression of GS. In FNH, the centrilobular zones of β- catenin activation are expanded [12], and this leads to an irregular, geographic distribution of GS overexpression [2]. In HCA and HCC, several diﬀerent patterns of GS expression have been observed, all distinct from that seen in FNH. As discussed later, activating mutations of the β- catenin gene occur in a subset of HCA. In adenomas with β-catenin mutation, GS is diﬀusely expressed, whereas in adenomas with normal β-catenin, GS is absent in large areas of the lesion. Unfortunately, however, in the latter type of HCA, irregular and/or centrizonal expression of GS can be present focally, particularly at the periphery of the tumor [2]. Therefore, although GS expression appears to robustly diﬀerentiate FNH from HCA in full resection specimens, it is less reliable for making this distinction in needle biopsies. 3. Hepatocellular Adenoma (HCA) First described a half-century ago, HCA is a rare, benign liver tumor that most commonly occurs in women and has been associated with oral contraceptive use [19]. It is a neoplasm of demonstrated clonal origin [20, 21] and has a small but nonnegligible risk of malignant transformation [22]. Hemorrhage, pain, and rupture are other, more frequent complications of HCA, and their likelihood is proportional to the size of the tumor [1]. Histologically, HCA is a proliferation of mature-appearing hepatocytes arranged in cords one to two cells thick and lacking portal tracts. Isolated arterioles surrounded by hepatocytes and lacking a ﬁbrous sheath (“naked” or “unpaired” arteries) can be seen scattered throughout the lesion. Arterioles accompanied by portal venules may be observed in a few portal-like structures, but these lack bile ducts. In some HCA, occasional isolated bile ductules may be seen, especially when immunohistochemical stains for CK7 or CK19 are applied. In 2002, Bluteau et al. published the results of a genome- wide search for tumor suppressor genes in HCA [23]. Microsatellite analysis revealed a loss of heterozygosity (a) (b) (c) 3 Figure 2: Patterns of immunohistochemical positivity for CK7 in benign hepatocellular lesions. (a) FNH displays strong CK7 staining of proliferating ductules but virtually no positivity within hepa- tocytes. (b) An example of HCA (nonmutated, noninﬂammatory type) showing focal, moderate CK7 staining within hepatocytes as well as heavy staining of a few bile ductules. (c) An I-HCA containing areas of bile ductular proliferation (arrow) with strong CK7 expression, resembling a ductular reaction in FNH, as well as other areas (e.g., bottom of image) in which there is focal hepatocellular staining for CK7, as is typical of HCA. (LOH) for markers at chromosome 12q caused by a small deletion of this region in ﬁve out of ten HCA. The TCF1 gene, encoding hepatocyte nuclear factor 1 (HNF1), was found to reside within the deletion. Because the function of HNF1 as a liver-speciﬁc transcription factor was already well established, this gene was pursued as the most likely candi- date tumor suppressor aﬀected by LOH in these adenomas. Further investigation conﬁrmed that in ten out of sixteen HCA analyzed, HNF1 was somatically inactivated, either through a combination of gene deletion and mutation or via bi-allelic mutation. Interestingly, germline mutation of the HNF1 gene had been previously discovered to underlie a rare form of familial noninsulin dependent diabetes, MODY3 (maturity-onset diabetes of the young, type 3) [24]. This form of diabetes is associated with an increased risk of hep- atic adenomatosis, due to random somatic inactivation of the second, nongermline-mutant HNF1 allele in the liver [25]. Shortly after genetic alterations of HNF1 in HCA were reported, a group in Taiwan published their discovery of activating mutations of the β-catenin gene in a fraction of these tumors. Earlier work by many independent groups had implicated Wnt pathway activation in liver carcinogenesis, and β-catenin mutations have been found in a signiﬁcant percentage of HCC. To investigate the possibility that aberrant β-catenin signaling plays a role in very early hepatic