In glioma, IDH1 mutations reduce a-ketoglutarate and accumulate 2-hydroxyglutarate which inhibits activity of TET 5-methylcytosine hydroxylases, leading to the decreased level of 5 hmC. In melanoma, the decreased 5 hmC plays a critical role in melanoma development. In addition, the decreased expressions of IDH2 and TET2 are responsible for the decreased 5 hmC and restoration of IDH2 or TET2 suppresses melanoma growth and increases tumor-free survival in animal models. These studies indicated that the altered 5 hmC may play an important role in pathogenesis of cancers. To determine whether the altered level of 5 hmC may occur in development of HCC, we investigated level of 5 hmC in an animal model with carcinogen DEN-induced liver cancer. This tumor model resembles human HCC because DEN induced chronic injure, inflammation, fibrogenesis and development of HCC. In this tumor model, we found that 5 hmC level was gradually decreased in liver during period of induction with DEN, as compared with normal liver tissues with high level of 5 hmC. Furthermore, we found that level of 5 hmC was further decreased in liver cancer tissues, as compared with non-tumor tissue. This data may support the notation that decrease of 5 hmC is a novel biomarker for development of HCC. DNA methylation was the first well-characterized epigenetic modification and has been demonstrated to play an important role in carcinogenesis. These studies have demonstrated that aberrant DNA methylation in cancer is not only associated with the repression of chromatin related to specific genes, but also with the repression of large chromosomal regions. Recently, the discovery of 5 hmC as a novel DNA modification marker occurred in AbMole Berbamine mammalian genomes has raised many questions regarding the role of this DNA demethylation in epigenetic regulation. The 5 mC oxidative pathway mediated by the TET proteins may be relevant for activation or repression of gene expression by associating with transcriptional repressors or activation factors. Recent studies have demonstrated that the altered 5 hmC was observed in different types of cancers and might play an important role in pathogenesis of cancers. However, it is not well known whether level of 5 hmC is changed in HCC and the altered 5 hmC is associated with outcome of patients with HCC. Thus, in this study we investigated alternation of 5 hmC in HCC from human patients and an animal model with carcinogen DEN-induced liver cancer. Our data showed that level of 5 hmC was significantly reduced in HCC tumor tissues, as compared with non-tumor tissues. This data further supported the previous observations of the reduced level of 5 hmC occurred in other types of cancers. Furthermore, we demonstrated that low level of 5 hmC in HCC was correlated with tumor size and AFP level. The most importantly, low level of 5 hmC in HCC was associated with short overall survival of patients with HCC. The recent study showed that there was a correlation between loss of 5 hmC and outcome of patients with melanoma. This data indicated that the decreased level of 5 hmC predicts poor prognosis of HCC patients. Furthermore, our data indicated that 5 mC level was increased in HCC tissues and the increased 5 mC level was associated with capsular invasion, vascular thrombosis, tumor recurrence and overall survival. The increased 5 mC level is coincident with the decreased 5 hmC level in HCC tissues. This data suggest that status of DNA methylation is based on balance of 5 mC and 5 hmC and DNA methylation is reversible in HCC tumor cells. In order to know whether the altered level of 5 hmC may occur in development of HCC, we investigated level of 5 hmC in an animal model with carcinogen DEN-induced liver cancer.