The hepatitis B virus (HBV) frequently integrates into hepatocellular genomic DNA during viral infection. Transcriptional transactivators encoded by integrated HBV X and 3' truncated preS/S sequences are known to stimulate gene expression from homologous and heterologous promoters. Here we demonstrate that 21 of 26 (81%) hepatocellular carcinoma tissues/cell lines contain coding sequences for at least one of the two known transactivators. Four integrated X and three preS/S transactivator sequences contained in five isolates from three hepatoma primary tissues or cell lines were used as examples to prove functionality of the encoded transactivators. In one case, where both X and preS/S sequences were present, dissection of X and preS/S transactivator sequences showed independent functionality. The investigation of X- and preS/S-specific RNA and protein expression revealed the existence of carboxyterminally truncated viral-cellular fusion proteins that were able to stimulate gene expression from the c-fos proto-oncogene promoter five- to ten-fold. These results demonstrate that structurally intact HBV transactivator sequences are integrated in the majority of HBV-associated HCCs/hepatoma cell lines. In all tested examples integrated DNAs had retained functionality as transactivators. This data thereby support indirectly the hypothesis of a possible involvement of HBV transactivators in liver cell proliferation and hepatocarcinogenesis.