p53 and p73 proteins activate similar target genes and induce apoptosis and cell cycle arrest. However, p53, but not p73 is considered a tumour-suppressor gene. Unlike p53, p73 de®ciency in mice does not lead to a cancerprone phenotype, and p73 gene is not mutated in human cancers, including hepatocellular carcinoma. Here we report that normal liver cells express only DN-p73 transcript forms giving rise to the synthesis of Nterminally truncated, transcriptionally inactive and dominant negative p73 proteins. In contrast, most hepatocellular carcinoma cells express TA-p73 transcript forms encoding full-length and transcriptionally active p73 proteins, in addition to DN-p73. We also show that together with the acquired expression of TA-p73, thè retinoblastoma pathway' is inactivated, and E2F1-target genes including cyclin E and p14 ARF are activated in hepatocellular carcinoma. However, there was no full correlation between`retinoblastoma pathway' inactivation and TA-p73 expression. Most TA-p73-expressing hepatocellular carcinoma cells have also lost p53 function either by lack of expression or missense mutations. The p73 gene, encoding only DN-p73 protein, may function as a tumour promoter rather than a tumour suppressor in liver tissue. This may be one reason why p73 is not a mutation target in hepatocellular carcinoma. Oncogene (2001) 20, 5111 ± 5117.