Three major molecular pathways have been identified altered in specific HCA subgroups that are defined by either (1) inactivation of hepatocyte nuclear factor 1A (HNF1A) transcription factor, (2) activation of the WNT/β-catenin by CTNNB1 mutations, or (3) activation of the IL6/STAT3 pathway by somatic mutation of IL6ST, GNAS, or STAT3. Here, we will review the different molecular classes of HCA. The gene discussed is GNAS; the disease is hepatocellular adenoma.