Transforming activity was detected in a hepatocellular carcinoma (HCC) carrying four integrated hepatitis B virus (HBV) DNA. This transforming gene was identified as hst-1, that lies in chromosome 11, band q13.3. One of the integrated HBV DNAs was found to lie close to the hst-1, and the hst-1 and the integrated HBV DNA were found to be co-amplified. The region of the amplification was limited. A model has been proposed that correlates the viral integration, amplification and activation of an oncogene.