Hepatocyte nuclear factor 4 alpha (HNF4α), the master regulator of hepatocyte differentiation, has been recently shown to inhibit hepatocyte proliferation by way of unknown mechanisms. We investigated the mechanisms of HNF4α-induced inhibition of hepatocyte proliferation using a novel tamoxifen (TAM)-inducible, hepatocyte-specific HNF4α knockdown mouse model. Hepatocyte-specific deletion of HNF4α in adult mice resulted in increased hepatocyte proliferation, with a significant increase in liver-to-body-weight ratio. We determined global gene expression changes using Illumina HiSeq-based RNA sequencing, which revealed that a significant number of up-regulated genes following deletion of HNF4α were associated with cancer pathogenesis, cell cycle control, and cell proliferation. The pathway analysis further revealed that c-Myc-regulated gene expression network was highly activated following HNF4α deletion. To determine whether deletion of HNF4α affects cancer pathogenesis, HNF4α knockdown was induced in mice treated with the known hepatic carcinogen diethylnitrosamine (DEN). Deletion of HNF4α significantly increased the number and size of DEN-induced hepatic tumors. Pathological analysis revealed that tumors in HNF4α-deleted mice were well-differentiated hepatocellular carcinoma (HCC) and mixed HCC-cholangiocarcinoma. Analysis of tumors and surrounding normal liver tissue in DEN-treated HNF4α knockout mice showed significant induction in c-Myc expression. Taken together, deletion of HNF4α in adult hepatocytes results in increased hepatocyte proliferation and promotion of DEN-induced hepatic tumors secondary to aberrant c-Myc activation. Copyright © 2013 American Association for the Study of Liver Diseases.