HBV DNAs are often found in integrated form in human hepatocellular carcinoma (HCC). Since transactivation of the X gene has been shown, surveys of collections of HBV integrants with flanking cellular sequence were performed to clarify whether they might exhibit a transactivation. The majority of integrants showed transactivation effect which may to be due to the virus-cell fusion products derived from the 3' truncated X gene. Additionally, it has been found that 3' truncated preS2/S gene in the integrant encodes a transactivator to which C terminal truncation is essential. These results suggest that the transactivating effect of integrated HBV DNAs plays a role in hepatocarcinogenesis by activating cellular genes.